Backchannel doesn't have the rq_task->tk_clientid pointer set.

Otherwise can lead to the following oops:
ocalhost login: [  111.385319] BUG: unable to handle kernel NULL pointer dereference at 0000000000000004
[  111.388073] #PF error: [normal kernel read fault]
[  111.389452] PGD 80000000290d8067 P4D 80000000290d8067 PUD 75f25067 PMD 0
[  111.391224] Oops: 0000 [#1] SMP PTI
[  111.392151] CPU: 0 PID: 3533 Comm: NFSv4 callback Not tainted 5.0.0-rc7+ #1
[  111.393787] Hardware name: VMware, Inc. VMware Virtual Platform/440BX Desktop Reference Platform, BIOS 6.00 07/02/2015
[  111.396340] RIP: 0010:trace_event_raw_event_xprt_enq_xmit+0x6f/0xf0 [sunrpc]
[  111.397974] Code: 00 00 00 48 89 ee 48 89 e7 e8 bd 0a 85 d7 48 85 c0 74 4a 41 0f b7 94 24 e0 00 00 00 48 89 e7 89 50 08 49 8b 94 24 a8 00 00 00 <8b> 52 04 89 50 0c 49 8b 94 24 c0 00 00 00 8b 92 a8 00 00 00 0f ca
[  111.402215] RSP: 0018:ffffb98743263cf8 EFLAGS: 00010286
[  111.403406] RAX: ffffa0890fc3bc88 RBX: 0000000000000003 RCX: 0000000000000000
[  111.405057] RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffffb98743263cf8
[  111.406656] RBP: ffffa0896f5368f0 R08: 0000000000000246 R09: 0000000000000000
[  111.408437] R10: ffffe19b01c01500 R11: 0000000000000000 R12: ffffa08977d28a00
[  111.410210] R13: 0000000000000004 R14: ffffa089315303f0 R15: ffffa08931530000
[  111.411856] FS:  0000000000000000(0000) GS:ffffa0897bc00000(0000) knlGS:0000000000000000
[  111.413699] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  111.415068] CR2: 0000000000000004 CR3: 000000002ac90004 CR4: 00000000001606f0
[  111.416745] Call Trace:
[  111.417339]  xprt_request_enqueue_transmit+0x2b6/0x4a0 [sunrpc]
[  111.418709]  ? rpc_task_need_encode+0x40/0x40 [sunrpc]
[  111.419957]  call_bc_transmit+0xd5/0x170 [sunrpc]
[  111.421067]  __rpc_execute+0x7e/0x3f0 [sunrpc]
[  111.422177]  rpc_run_bc_task+0x78/0xd0 [sunrpc]
[  111.423212]  bc_svc_process+0x281/0x340 [sunrpc]
[  111.424325]  nfs41_callback_svc+0x130/0x1c0 [nfsv4]
[  111.425430]  ? remove_wait_queue+0x60/0x60
[  111.426398]  kthread+0xf5/0x130
[  111.427155]  ? nfs_callback_authenticate+0x50/0x50 [nfsv4]
[  111.428388]  ? kthread_bind+0x10/0x10
[  111.429270]  ret_from_fork+0x1f/0x30

localhost login: [  467.462259] BUG: unable to handle kernel NULL pointer dereference at 0000000000000004
[  467.464411] #PF error: [normal kernel read fault]
[  467.465445] PGD 80000000728c1067 P4D 80000000728c1067 PUD 728c0067 PMD 0
[  467.466980] Oops: 0000 [#1] SMP PTI
[  467.467759] CPU: 0 PID: 3517 Comm: NFSv4 callback Not tainted 5.0.0-rc7+ #1
[  467.469393] Hardware name: VMware, Inc. VMware Virtual Platform/440BX Desktop Reference Platform, BIOS 6.00 07/02/2015
[  467.471840] RIP: 0010:trace_event_raw_event_xprt_transmit+0x7c/0xf0 [sunrpc]
[  467.473392] Code: f6 48 85 c0 74 4b 49 8b 94 24 98 00 00 00 48 89 e7 0f b7 92 e0 00 00 00 89 50 08 49 8b 94 24 98 00 00 00 48 8b 92 a8 00 00 00 <8b> 52 04 89 50 0c 41 8b 94 24 a8 00 00 00 0f ca 89 50 10 41 8b 94
[  467.477605] RSP: 0018:ffffabe7434fbcd0 EFLAGS: 00010282
[  467.478793] RAX: ffff99720fc3bce0 RBX: 0000000000000003 RCX: 0000000000000000
[  467.480409] RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffffabe7434fbcd0
[  467.482011] RBP: ffff99726f631948 R08: 0000000000000246 R09: 0000000000000000
[  467.483591] R10: 0000000070000000 R11: 0000000000000000 R12: ffff997277dfcc00
[  467.485226] R13: 0000000000000000 R14: 0000000000000000 R15: ffff99722fecdca8
[  467.486830] FS:  0000000000000000(0000) GS:ffff99727bc00000(0000) knlGS:0000000000000000
[  467.488596] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  467.489931] CR2: 0000000000000004 CR3: 00000000270e6006 CR4: 00000000001606f0
[  467.491559] Call Trace:
[  467.492128]  xprt_transmit+0x303/0x3f0 [sunrpc]
[  467.493143]  ? rpc_task_need_encode+0x40/0x40 [sunrpc]
[  467.494328]  call_bc_transmit+0x49/0x170 [sunrpc]
[  467.495379]  __rpc_execute+0x7e/0x3f0 [sunrpc]
[  467.496451]  rpc_run_bc_task+0x78/0xd0 [sunrpc]
[  467.497467]  bc_svc_process+0x281/0x340 [sunrpc]
[  467.498507]  nfs41_callback_svc+0x130/0x1c0 [nfsv4]
[  467.499751]  ? remove_wait_queue+0x60/0x60
[  467.500686]  kthread+0xf5/0x130
[  467.501438]  ? nfs_callback_authenticate+0x50/0x50 [nfsv4]
[  467.502640]  ? kthread_bind+0x10/0x10
[  467.503454]  ret_from_fork+0x1f/0x30
Signed-off-by: Olga Kornievskaia <kolga@netapp.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>