md/raid1: avoid soft lockup under high load

As all I/O is being pushed through a kernel thread the softlockup
watchdog might be triggered under high load.
Signed-off-by: NHannes Reinecke <hare@suse.de>
Signed-off-by: NSong Liu <songliubraving@fb.com>

drivers/md/raid1.c

@@ -819,6 +819,7 @@ static void flush_bio_list(struct r1conf *conf, struct bio *bio)
 		else
 			generic_make_request(bio);
 		bio = next;
+		cond_resched();
 	}
 }