scsi: core: Avoid that system resume triggers a kernel warning

scsi_device_quiesce() and scsi_device_resume() are called during
system-wide suspend and resume. scsi_device_quiesce() only succeeds for
SCSI devices that are in one of the RUNNING, OFFLINE or TRANSPORT_OFFLINE
states (see also scsi_set_device_state()). This patch avoids that the
following warning is triggered when resuming a system for which quiescing a
SCSI device failed:

WARNING: CPU: 2 PID: 11303 at drivers/scsi/scsi_lib.c:2600 scsi_device_resume+0x4f/0x58
CPU: 2 PID: 11303 Comm: kworker/u8:70 Not tainted 5.0.0-rc1+ #50
Hardware name: LENOVO 80E3/Lancer 5B2, BIOS A2CN45WW(V2.13) 08/04/2016
Workqueue: events_unbound async_run_entry_fn
Call Trace:
scsi_dev_type_resume+0x2e/0x60
async_run_entry_fn+0x32/0xd8
process_one_work+0x1f4/0x420
worker_thread+0x28/0x3c0
kthread+0x118/0x130
ret_from_fork+0x22/0x40

Cc: Przemek Socha <[e-mail]>
Reported-by: Przemek Socha <[e-mail]>
Fixes: 3a0a529971ec ("block, scsi: Make SCSI quiesce and resume work reliably") # v4.15
Signed-off-by: Bart Van Assche <[e-mail]>
Signed-off-by: Martin K. Petersen <[e-mail]>

drivers/scsi/scsi_lib.c

···
 	 * device deleted during suspend)
 	 */
 	mutex_lock(&sdev->state_mutex);
-	WARN_ON_ONCE(!sdev->quiesced_by);
 	sdev->quiesced_by = NULL;
 	blk_clear_pm_only(sdev->request_queue);
 	if (sdev->sdev_state == SDEV_QUIESCE)