mld_{query | report}_work() processes queued events.
If there are too many events in the queue, it re-queue a work.
And then, it returns without in6_dev_put().
But if queuing is failed, it should call in6_dev_put(), but it doesn't.
So, a reference count leak would occur.

THREAD0				THREAD1
mld_report_work()
				spin_lock_bh()
				if (!mod_delayed_work())
					in6_dev_hold();
				spin_unlock_bh()
	spin_lock_bh()
	schedule_delayed_work()
	spin_unlock_bh()

Script to reproduce(by Hangbin Liu):
   ip netns add ns1
   ip netns add ns2
   ip netns exec ns1 sysctl -w net.ipv6.conf.all.force_mld_version=1
   ip netns exec ns2 sysctl -w net.ipv6.conf.all.force_mld_version=1

   ip -n ns1 link add veth0 type veth peer name veth0 netns ns2
   ip -n ns1 link set veth0 up
   ip -n ns2 link set veth0 up

   for i in `seq 50`; do
           for j in `seq 100`; do
                   ip -n ns1 addr add 2021:${i}::${j}/64 dev veth0
                   ip -n ns2 addr add 2022:${i}::${j}/64 dev veth0
           done
   done
   modprobe -r veth
   ip -a netns del

splat looks like:
 unregister_netdevice: waiting for veth0 to become free. Usage count = 2
 leaked reference.
  ipv6_add_dev+0x324/0xec0
  addrconf_notify+0x481/0xd10
  raw_notifier_call_chain+0xe3/0x120
  call_netdevice_notifiers+0x106/0x160
  register_netdevice+0x114c/0x16b0
  veth_newlink+0x48b/0xa50 [veth]
  rtnl_newlink+0x11a2/0x1a40
  rtnetlink_rcv_msg+0x63f/0xc00
  netlink_rcv_skb+0x1df/0x3e0
  netlink_unicast+0x5de/0x850
  netlink_sendmsg+0x6c9/0xa90
  ____sys_sendmsg+0x76a/0x780
  __sys_sendmsg+0x27c/0x340
  do_syscall_64+0x43/0x90
  entry_SYSCALL_64_after_hwframe+0x63/0xcd
Tested-by: NHangbin Liu <liuhangbin@gmail.com>
Fixes: f185de28 ("mld: add new workqueues for process mld events")
Signed-off-by: NTaehee Yoo <ap420073@gmail.com>
Reviewed-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>