IB/srp: Fix race conditions related to task management

Avoid that srp_process_rsp() overwrites the status information
in ch if the SRP target response timed out and processing of
another task management function has already started. Avoid that
issuing multiple task management functions concurrently triggers
list corruption. This patch prevents that the following stack
trace appears in the system log:

WARNING: CPU: 8 PID: 9269 at lib/list_debug.c:52 __list_del_entry_valid+0xbc/0xc0
list_del corruption. prev->next should be ffffc90004bb7b00, but was ffff8804052ecc68
CPU: 8 PID: 9269 Comm: sg_reset Tainted: G        W       4.10.0-rc7-dbg+ #3
Call Trace:
 dump_stack+0x68/0x93
 __warn+0xc6/0xe0
 warn_slowpath_fmt+0x4a/0x50
 __list_del_entry_valid+0xbc/0xc0
 wait_for_completion_timeout+0x12e/0x170
 srp_send_tsk_mgmt+0x1ef/0x2d0 [ib_srp]
 srp_reset_device+0x5b/0x110 [ib_srp]
 scsi_ioctl_reset+0x1c7/0x290
 scsi_ioctl+0x12a/0x420
 sd_ioctl+0x9d/0x100
 blkdev_ioctl+0x51e/0x9f0
 block_ioctl+0x38/0x40
 do_vfs_ioctl+0x8f/0x700
 SyS_ioctl+0x3c/0x70
 entry_SYSCALL_64_fastpath+0x18/0xad

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Cc: Israel Rukshin <israelr@mellanox.com>
Cc: Max Gurtovoy <maxg@mellanox.com>
Cc: Laurence Oberman <loberman@redhat.com>
Cc: Steve Feeley <Steve.Feeley@sandisk.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>