RDMA/mlx5: Fix access to wrong pointer while performing flush due to error

RDMA/mlx5: Fix access to wrong pointer while performing flush due to error

commit 950bf4f17725556bbc773a5b71e88a6c14c9ff25 upstream.

The main difference between send and receive SW completions is related to
separate treatment of WQ queue. For receive completions, the initial index
to be flushed is stored in "tail", while for send completions, it is in
deleted "last_poll".

  CPU: 54 PID: 53405 Comm: kworker/u161:0 Kdump: loaded Tainted: G           OE    --------- -t - 4.18.0-147.el8.ppc64le #1
  Workqueue: ib-comp-unb-wq ib_cq_poll_work [ib_core]
  NIP:  c000003c7c00a000 LR: c00800000e586af4 CTR: c000003c7c00a000
  REGS: c0000036cc9db940 TRAP: 0400   Tainted: G           OE    --------- -t -  (4.18.0-147.el8.ppc64le)
  MSR:  9000000010009033 <SF,HV,EE,ME,IR,DR,RI,LE>  CR: 24004488  XER: 20040000
  CFAR: c00800000e586af0 IRQMASK: 0
  GPR00: c00800000e586ab4 c0000036cc9dbbc0 c00800000e5f1a00 c0000037d8433800
  GPR04: c000003895a26800 c0000037293f2000 0000000000000201 0000000000000011
  GPR08: c000003895a26c80 c000003c7c00a000 0000000000000000 c00800000ed30438
  GPR12: c000003c7c00a000 c000003fff684b80 c00000000017c388 c00000396ec4be40
  GPR16: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
  GPR20: c00000000151e498 0000000000000010 c000003895a26848 0000000000000010
  GPR24: 0000000000000010 0000000000010000 c000003895a26800 0000000000000000
  GPR28: 0000000000000010 c0000037d8433800 c000003895a26c80 c000003895a26800
  NIP [c000003c7c00a000] 0xc000003c7c00a000
  LR [c00800000e586af4] __ib_process_cq+0xec/0x1b0 [ib_core]
  Call Trace:
  [c0000036cc9dbbc0] [c00800000e586ab4] __ib_process_cq+0xac/0x1b0 [ib_core] (unreliable)
  [c0000036cc9dbc40] [c00800000e586c88] ib_cq_poll_work+0x40/0xb0 [ib_core]
  [c0000036cc9dbc70] [c000000000171f44] process_one_work+0x2f4/0x5c0
  [c0000036cc9dbd10] [c000000000172a0c] worker_thread+0xcc/0x760
  [c0000036cc9dbdc0] [c00000000017c52c] kthread+0x1ac/0x1c0
  [c0000036cc9dbe30] [c00000000000b75c] ret_from_kernel_thread+0x5c/0x80

Fixes: 8e3b68830186 ("RDMA/mlx5: Delete unreachable handle_atomic code by simplifying SW completion")
Link: https://lore.kernel.org/r/20200318091640.44069-1-leon@kernel.org
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>