md: handle writes to broken raid10 arrays gracefully

When writing to a broken array, raid10 currently happily emits empty bio
lists.  IOW, the master bio will never be completed, sending writers to
UNINTERRUPTIBLE_SLEEP forever.

Signed-off-by: Arne Redlich <agr@powerkom-dd.de>
Acked-by: Neil Brown <neilb@suse.de>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/md/raid10.c

@@ -917,6 +917,13 @@ static int make_request(struct request_queue *q, struct bio * bio)
 		bio_list_add(&bl, mbio);
 	}
 
+	if (unlikely(!atomic_read(&r10_bio->remaining))) {
+		/* the array is dead */
+		md_write_end(mddev);
+		raid_end_bio_io(r10_bio);
+		return 0;
+	}
+
 	bitmap_startwrite(mddev->bitmap, bio->bi_sector, r10_bio->sectors, 0);
 	spin_lock_irqsave(&conf->device_lock, flags);
 	bio_list_merge(&conf->pending_bio_list, &bl);