RDMA/mlx5: Fix NULL pointer dereference in destroy_prefetch_work

RDMA/mlx5: Fix NULL pointer dereference in destroy_prefetch_work

q_deferred_work isn't initialized when creating an explicit ODP memory
region. This can lead to a NULL pointer dereference when user performs
asynchronous prefetch MR. Fix it by initializing q_deferred_work for
explicit ODP.

  BUG: kernel NULL pointer dereference, address: 0000000000000000
  #PF: supervisor read access in kernel mode
  #PF: error_code(0x0000) - not-present page
  PGD 0 P4D 0
  Oops: 0000 [#1] SMP PTI
  CPU: 4 PID: 6074 Comm: kworker/u16:6 Not tainted 5.7.0-rc1-for-upstream-perf-2020-04-17_07-03-39-64 #1
  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.12.1-0-ga5cab58e9a3f-prebuilt.qemu.org 04/01/2014
  Workqueue: events_unbound mlx5_ib_prefetch_mr_work [mlx5_ib]
  RIP: 0010:__wake_up_common+0x49/0x120
  Code: 04 89 54 24 0c 89 4c 24 08 74 0a 41 f6 01 04 0f 85 8e 00 00 00 48 8b 47 08 48 83 e8 18 4c 8d 67 08 48 8d 50 18 49 39 d4 74 66 <48> 8b 70 18 31 db 4c 8d 7e e8 eb 17 49 8b 47 18 48 8d 50 e8 49 8d
  RSP: 0000:ffffc9000097bd88 EFLAGS: 00010082
  RAX: ffffffffffffffe8 RBX: ffff888454cd9f90 RCX: 0000000000000000
  RDX: 0000000000000000 RSI: 0000000000000003 RDI: ffff888454cd9f90
  RBP: ffffc9000097bdd0 R08: 0000000000000000 R09: ffffc9000097bdd0
  R10: 0000000000000000 R11: 0000000000000001 R12: ffff888454cd9f98
  R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000003
  FS:  0000000000000000(0000) GS:ffff88846fd00000(0000) knlGS:0000000000000000
  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  CR2: 0000000000000000 CR3: 000000044c19e002 CR4: 0000000000760ee0
  DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
  DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
  PKRU: 55555554
  Call Trace:
   __wake_up_common_lock+0x7a/0xc0
   destroy_prefetch_work+0x5a/0x60 [mlx5_ib]
   mlx5_ib_prefetch_mr_work+0x64/0x80 [mlx5_ib]
   process_one_work+0x15b/0x360
   worker_thread+0x49/0x3d0
   kthread+0xf5/0x130
   ? rescuer_thread+0x310/0x310
   ? kthread_bind+0x10/0x10
   ret_from_fork+0x1f/0x30

Fixes: de5ed007a03d ("IB/mlx5: Fix implicit ODP race")
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20200521072504.567406-1-leon@kernel.org
Signed-off-by: Maor Gottlieb <maorg@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>