SUNRPC: Fix oops when trace sunrpc_task events in nfs client

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: Ditang Chen <chendt.fnst@cn.fujitsu.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>