md/raid1/raid10: add a cond_resched

During 'check' of a raid1 or raid10 it is possible for the management
thread to spend a lot of time running 'memcmp' on blocks from
different devices, so make sure the thread has a chance to schedule.
raid5d already has a cond_resched (in process_stripe).

Reported-By: Lee Howard <faxguy@howardsilvan.com>
Signed-off-by: NeilBrown <neilb@suse.de>
diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
index 51c4c5c..69fc76c 100644
--- a/drivers/md/raid10.c
+++ b/drivers/md/raid10.c
@@ -1632,6 +1632,7 @@
 				generic_make_request(bio);
 			}
 		}
+		cond_resched();
 	}
 	if (unplug)
 		unplug_slaves(mddev);