When tracking sunrpc_task events in nfs client, the clnt pointer may be NULL.

[  139.269266] BUG: unable to handle kernel NULL pointer dereference at 0000000000000004
[  139.269915] IP: [<ffffffffa026f216>] ftrace_raw_event_rpc_task_running+0x86/0xf0 [sunrpc]
[  139.269915] PGD 1d293067 PUD 1d294067 PMD 0
[  139.269915] Oops: 0000 [#1] SMP
[  139.269915] Modules linked in: nfsv4 dns_resolver nfs lockd sunrpc fscache sg ppdev e1000
serio_raw pcspkr parport_pc parport i2c_piix4 i2c_core microcode xfs libcrc32c sd_mod sr_mod
cdrom ata_generic crc_t10dif crct10dif_common pata_acpi ahci libahci ata_piix libata dm_mirror
dm_region_hash dm_log dm_mod
[  139.269915] CPU: 0 PID: 59 Comm: kworker/0:2 Not tainted 3.10.0-84.el7.x86_64 #1
[  139.269915] Hardware name: innotek GmbH VirtualBox/VirtualBox, BIOS VirtualBox 12/01/2006
[  139.269915] Workqueue: rpciod rpc_async_schedule [sunrpc]
[  139.269915] task: ffff88001b598000 ti: ffff88001b632000 task.ti: ffff88001b632000
[  139.269915] RIP: 0010:[<ffffffffa026f216>]  [<ffffffffa026f216>] ftrace_raw_event_rpc_task_running+0x86/0xf0 [sunrpc]
[  139.269915] RSP: 0018:ffff88001b633d70  EFLAGS: 00010206
[  139.269915] RAX: ffff88001dfc5338 RBX: ffff88001cc37a00 RCX: ffff88001dfc5334
[  139.269915] RDX: ffff88001dfc5338 RSI: 0000000000000000 RDI: ffff88001dfc533c
[  139.269915] RBP: ffff88001b633db0 R08: 000000000000002c R09: 000000000000000a
[  139.269915] R10: 0000000000062180 R11: 00000020759fb9dc R12: ffffffffa0292c20
[  139.269915] R13: ffff88001dfc5334 R14: 0000000000000000 R15: 0000000000000000
[  139.269915] FS:  0000000000000000(0000) GS:ffff88001fc00000(0000) knlGS:0000000000000000
[  139.269915] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
[  139.269915] CR2: 0000000000000004 CR3: 000000001d290000 CR4: 00000000000006f0
[  139.269915] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[  139.269915] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
[  139.269915] Stack:
[  139.269915]  000000001b633d98 0000000000000246 ffff88001df1dc00 ffff88001cc37a00
[  139.269915]  ffff88001bc35e60 0000000000000000 ffff88001ffa0a48 ffff88001bc35ee0
[  139.269915]  ffff88001b633e08 ffffffffa02704b5 0000000000010000 ffff88001cc37a70
[  139.269915] Call Trace:
[  139.269915]  [<ffffffffa02704b5>] __rpc_execute+0x1d5/0x400 [sunrpc]
[  139.269915]  [<ffffffffa0270706>] rpc_async_schedule+0x26/0x30 [sunrpc]
[  139.269915]  [<ffffffff8107867b>] process_one_work+0x17b/0x460
[  139.269915]  [<ffffffff8107942b>] worker_thread+0x11b/0x400
[  139.269915]  [<ffffffff81079310>] ? rescuer_thread+0x3e0/0x3e0
[  139.269915]  [<ffffffff8107fc80>] kthread+0xc0/0xd0
[  139.269915]  [<ffffffff8107fbc0>] ? kthread_create_on_node+0x110/0x110
[  139.269915]  [<ffffffff815d122c>] ret_from_fork+0x7c/0xb0
[  139.269915]  [<ffffffff8107fbc0>] ? kthread_create_on_node+0x110/0x110
[  139.269915] Code: 4c 8b 45 c8 48 8d 7d d0 89 4d c4 41 89 c9 b9 28 00 00 00 e8 9d b4 e9
e0 48 85 c0 49 89 c5 74 a2 48 89 c7 e8 9d 3f e9 e0 48 89 c2 <41> 8b 46 04 48 8b 7d d0 4c
89 e9 4c 89 e6 89 42 0c 0f b7 83 d4
[  139.269915] RIP  [<ffffffffa026f216>] ftrace_raw_event_rpc_task_running+0x86/0xf0 [sunrpc]
[  139.269915]  RSP <ffff88001b633d70>
[  139.269915] CR2: 0000000000000004
[  140.946406] ---[ end trace ba486328b98d7622 ]---
Signed-off-by: NDitang Chen <chendt.fnst@cn.fujitsu.com>
Signed-off-by: NTrond Myklebust <trond.myklebust@primarydata.com>

Signed-off-by: NTrond Myklebust <trond.myklebust@primarydata.com>

Instead of the pointer values, use the task and client identifier values
for tracing purposes.
Signed-off-by: NTrond Myklebust <Trond.Myklebust@netapp.com>

Add client side debugging to help trace socket connection/disconnection
and unexpected state change issues.
Signed-off-by: NTrond Myklebust <Trond.Myklebust@netapp.com>

This patch adds three trace points to the status routines
in the sunrpc state machine.

The goal of these trace points is to give an Admin
the ability to check on binding status or connection
status to see if there is a potential problem.
Signed-off-by: NSteve Dickson <steved@redhat.com>
Signed-off-by: NTrond Myklebust <Trond.Myklebust@netapp.com>

The reporting of the RPC queue name needs to use the __string()
event interface.
Reported-by: NNeil Horman <nhorman@redhat.com>
Signed-off-by: NTrond Myklebust <Trond.Myklebust@netapp.com>

Add declarations to allow tracing of RPC call creation, running, sleeping,
and destruction.
Signed-off-by: NTrond Myklebust <Trond.Myklebust@netapp.com>