[SCSI] Fix race between starved list and device removal

scsi_run_queue() examines all SCSI devices that are present on
the starved list. Since scsi_run_queue() unlocks the SCSI host
lock a SCSI device can get removed after it has been removed
from the starved list and before its queue is run. Protect
against that race condition by holding a reference on the
queue while running it.

Reported-by: Chanho Min <chanho.min@lge.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>