dm: add cond_resched() to dm_wq_requeue_work()

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

Fixes: 8b211aa ("dm: add two stage requeue mechanism")
Cc: stable@vger.kernel.org
Signed-off-by: Mike Snitzer <snitzer@kernel.org>

Author: Mike Snitzer

drivers/md/dm.c

@@ -1009,6 +1009,7 @@ static void dm_wq_requeue_work(struct work_struct *work)
 		io->next = NULL;
 		__dm_io_complete(io, false);
 		io = next;
+		cond_resched();
 	}
 }