__gre6_xmit() pushes GRE headers before handing over to ip6_tnl_xmit()
for generic IP-in-IP processing. However it doesn't make sure that there
is enough headroom to push the header to. That can lead to the panic
cited below. (Reproducer below that).

Fix by requesting either needed_headroom if already primed, or just the
bare minimum needed for the header otherwise.

[  158.576725] kernel BUG at net/core/skbuff.c:104!
[  158.581510] invalid opcode: 0000 [#1] PREEMPT SMP KASAN PTI
[  158.587174] Modules linked in: act_mirred cls_matchall ip6_gre ip6_tunnel tunnel6 gre sch_ingress vrf veth x86_pkg_temp_thermal mlx_platform nfsd e1000e leds_mlxcpld
[  158.602268] CPU: 1 PID: 16 Comm: ksoftirqd/1 Not tainted 4.17.0-rc4-net_master-custom-139 #10
[  158.610938] Hardware name: Mellanox Technologies Ltd. "MSN2410-CB2F"/"SA000874", BIOS 4.6.5 03/08/2016
[  158.620426] RIP: 0010:skb_panic+0xc3/0x100
[  158.624586] RSP: 0018:ffff8801d3f27110 EFLAGS: 00010286
[  158.629882] RAX: 0000000000000082 RBX: ffff8801c02cc040 RCX: 0000000000000000
[  158.637127] RDX: 0000000000000082 RSI: dffffc0000000000 RDI: ffffed003a7e4e18
[  158.644366] RBP: ffff8801bfec8020 R08: ffffed003aabce19 R09: ffffed003aabce19
[  158.651574] R10: 000000000000000b R11: ffffed003aabce18 R12: ffff8801c364de66
[  158.658786] R13: 000000000000002c R14: 00000000000000c0 R15: ffff8801c364de68
[  158.666007] FS:  0000000000000000(0000) GS:ffff8801d5400000(0000) knlGS:0000000000000000
[  158.674212] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  158.680036] CR2: 00007f4b3702dcd0 CR3: 0000000003228002 CR4: 00000000001606e0
[  158.687228] Call Trace:
[  158.689752]  ? __gre6_xmit+0x246/0xd80 [ip6_gre]
[  158.694475]  ? __gre6_xmit+0x246/0xd80 [ip6_gre]
[  158.699141]  skb_push+0x78/0x90
[  158.702344]  __gre6_xmit+0x246/0xd80 [ip6_gre]
[  158.706872]  ip6gre_tunnel_xmit+0x3bc/0x610 [ip6_gre]
[  158.711992]  ? __gre6_xmit+0xd80/0xd80 [ip6_gre]
[  158.716668]  ? debug_check_no_locks_freed+0x210/0x210
[  158.721761]  ? print_irqtrace_events+0x120/0x120
[  158.726461]  ? sched_clock_cpu+0x18/0x210
[  158.730572]  ? sched_clock_cpu+0x18/0x210
[  158.734692]  ? cyc2ns_read_end+0x10/0x10
[  158.738705]  ? skb_network_protocol+0x76/0x200
[  158.743216]  ? netif_skb_features+0x1b2/0x550
[  158.747648]  dev_hard_start_xmit+0x137/0x770
[  158.752010]  sch_direct_xmit+0x2ef/0x5d0
[  158.755992]  ? pfifo_fast_dequeue+0x3fa/0x670
[  158.760460]  ? pfifo_fast_change_tx_queue_len+0x810/0x810
[  158.765975]  ? __lock_is_held+0xa0/0x160
[  158.770002]  __qdisc_run+0x39e/0xfc0
[  158.773673]  ? _raw_spin_unlock+0x29/0x40
[  158.777781]  ? pfifo_fast_enqueue+0x24b/0x3e0
[  158.782191]  ? sch_direct_xmit+0x5d0/0x5d0
[  158.786372]  ? pfifo_fast_dequeue+0x670/0x670
[  158.790818]  ? __dev_queue_xmit+0x172/0x1770
[  158.795195]  ? preempt_count_sub+0xf/0xd0
[  158.799313]  __dev_queue_xmit+0x410/0x1770
[  158.803512]  ? ___slab_alloc+0x605/0x930
[  158.807525]  ? ___slab_alloc+0x605/0x930
[  158.811540]  ? memcpy+0x34/0x50
[  158.814768]  ? netdev_pick_tx+0x1c0/0x1c0
[  158.818895]  ? __skb_clone+0x2fd/0x3d0
[  158.822712]  ? __copy_skb_header+0x270/0x270
[  158.827079]  ? rcu_read_lock_sched_held+0x93/0xa0
[  158.831903]  ? kmem_cache_alloc+0x344/0x4d0
[  158.836199]  ? skb_clone+0x123/0x230
[  158.839869]  ? skb_split+0x820/0x820
[  158.843521]  ? tcf_mirred+0x554/0x930 [act_mirred]
[  158.848407]  tcf_mirred+0x554/0x930 [act_mirred]
[  158.853104]  ? tcf_mirred_act_wants_ingress.part.2+0x10/0x10 [act_mirred]
[  158.860005]  ? __lock_acquire+0x706/0x26e0
[  158.864162]  ? mark_lock+0x13d/0xb40
[  158.867832]  tcf_action_exec+0xcf/0x2a0
[  158.871736]  tcf_classify+0xfa/0x340
[  158.875402]  __netif_receive_skb_core+0x8e1/0x1c60
[  158.880334]  ? nf_ingress+0x500/0x500
[  158.884059]  ? process_backlog+0x347/0x4b0
[  158.888241]  ? lock_acquire+0xd8/0x320
[  158.892050]  ? process_backlog+0x1b6/0x4b0
[  158.896228]  ? process_backlog+0xc2/0x4b0
[  158.900291]  process_backlog+0xc2/0x4b0
[  158.904210]  net_rx_action+0x5cc/0x980
[  158.908047]  ? napi_complete_done+0x2c0/0x2c0
[  158.912525]  ? rcu_read_unlock+0x80/0x80
[  158.916534]  ? __lock_is_held+0x34/0x160
[  158.920541]  __do_softirq+0x1d4/0x9d2
[  158.924308]  ? trace_event_raw_event_irq_handler_exit+0x140/0x140
[  158.930515]  run_ksoftirqd+0x1d/0x40
[  158.934152]  smpboot_thread_fn+0x32b/0x690
[  158.938299]  ? sort_range+0x20/0x20
[  158.941842]  ? preempt_count_sub+0xf/0xd0
[  158.945940]  ? schedule+0x5b/0x140
[  158.949412]  kthread+0x206/0x300
[  158.952689]  ? sort_range+0x20/0x20
[  158.956249]  ? kthread_stop+0x570/0x570
[  158.960164]  ret_from_fork+0x3a/0x50
[  158.963823] Code: 14 3e ff 8b 4b 78 55 4d 89 f9 41 56 41 55 48 c7 c7 a0 cf db 82 41 54 44 8b 44 24 2c 48 8b 54 24 30 48 8b 74 24 20 e8 16 94 13 ff <0f> 0b 48 c7 c7 60 8e 1f 85 48 83 c4 20 e8 55 ef a6 ff 89 74 24
[  158.983235] RIP: skb_panic+0xc3/0x100 RSP: ffff8801d3f27110
[  158.988935] ---[ end trace 5af56ee845aa6cc8 ]---
[  158.993641] Kernel panic - not syncing: Fatal exception in interrupt
[  159.000176] Kernel Offset: disabled
[  159.003767] ---[ end Kernel panic - not syncing: Fatal exception in interrupt ]---

Reproducer:

	ip link add h1 type veth peer name swp1
	ip link add h3 type veth peer name swp3

	ip link set dev h1 up
	ip address add 192.0.2.1/28 dev h1

	ip link add dev vh3 type vrf table 20
	ip link set dev h3 master vh3
	ip link set dev vh3 up
	ip link set dev h3 up

	ip link set dev swp3 up
	ip address add dev swp3 2001:db8:2::1/64

	ip link set dev swp1 up
	tc qdisc add dev swp1 clsact

	ip link add name gt6 type ip6gretap \
		local 2001:db8:2::1 remote 2001:db8:2::2
	ip link set dev gt6 up

	sleep 1

	tc filter add dev swp1 ingress pref 1000 matchall skip_hw \
		action mirred egress mirror dev gt6
	ping -I h1 192.0.2.2

Fixes: c12b395a ("gre: Support GRE over IPv6")
Signed-off-by: NPetr Machata <petrm@mellanox.com>
Acked-by: NWilliam Tu <u9012063@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>