dm: add cond_resched() to dm_wq_work()

Otherwise the while() loop in dm_wq_work() can result in a "dead
loop" on systems that have preemption disabled. This is particularly
problematic on single cpu systems.

Cc: stable@vger.kernel.org
Signed-off-by: Pingfan Liu <piliu@redhat.com>
Acked-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>

drivers/md/dm.c

@@ -2570,6 +2570,7 @@ static void dm_wq_work(struct work_struct *work)
 			break;
 
 		submit_bio_noacct(bio);
+		cond_resched();
 	}
 }