RDMA/srp: Set scmnd->result only when scmnd is not NULL

RDMA/srp: Set scmnd->result only when scmnd is not NULL

[ Upstream commit 12f35199a2c0551187edbf8eb01379f0598659fa ]

This change fixes the following kernel NULL pointer dereference
which is reproduced by blktests srp/007 occasionally.

BUG: kernel NULL pointer dereference, address: 0000000000000170
PGD 0 P4D 0
Oops: 0002 [#1] PREEMPT SMP NOPTI
CPU: 0 PID: 9 Comm: kworker/0:1H Kdump: loaded Not tainted 6.0.0-rc1+ #37
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.15.0-29-g6a62e0cb0dfe-prebuilt.qemu.org 04/01/2014
Workqueue:  0x0 (kblockd)
RIP: 0010:srp_recv_done+0x176/0x500 [ib_srp]
Code: 00 4d 85 ff 0f 84 52 02 00 00 48 c7 82 80 02 00 00 00 00 00 00 4c 89 df 4c 89 14 24 e8 53 d3 4a f6 4c 8b 14 24 41 0f b6 42 13 <41> 89 87 70 01 00 00 41 0f b6 52 12 f6 c2 02 74 44 41 8b 42 1c b9
RSP: 0018:ffffaef7c0003e28 EFLAGS: 00000282
RAX: 0000000000000000 RBX: ffff9bc9486dea60 RCX: 0000000000000000
RDX: 0000000000000102 RSI: ffffffffb76bbd0e RDI: 00000000ffffffff
RBP: ffff9bc980099a00 R08: 0000000000000001 R09: 0000000000000001
R10: ffff9bca53ef0000 R11: ffff9bc980099a10 R12: ffff9bc956e14000
R13: ffff9bc9836b9cb0 R14: ffff9bc9557b4480 R15: 0000000000000000
FS:  0000000000000000(0000) GS:ffff9bc97ec00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000170 CR3: 0000000007e04000 CR4: 00000000000006f0
Call Trace:
 <IRQ>
 __ib_process_cq+0xb7/0x280 [ib_core]
 ib_poll_handler+0x2b/0x130 [ib_core]
 irq_poll_softirq+0x93/0x150
 __do_softirq+0xee/0x4b8
 irq_exit_rcu+0xf7/0x130
 sysvec_apic_timer_interrupt+0x8e/0xc0
 </IRQ>

Fixes: ad215aaea4f9 ("RDMA/srp: Make struct scsi_cmnd and struct srp_request adjacent")
Link: https://lore.kernel.org/r/20220831081626.18712-1-yangx.jy@fujitsu.com
Signed-off-by: Xiao Yang <yangx.jy@fujitsu.com>
Acked-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>