[PATCH] md: ensure bitmap_writeback_daemon handles shutdown properly.

mddev->bitmap gets clearred before the writeback daemon is stopped.  So the
write_back daemon needs to be careful not to dereference the 'bitmap' if it is
NULL.
Signed-off-by: NNeil Brown <neilb@cse.unsw.edu.au>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>

drivers/md/bitmap.c

@@ -1156,6 +1156,9 @@ static void bitmap_writeback_daemon(mddev_t *mddev)
 		err = -EINTR;
 		goto out;
 	}
+	if (bitmap == NULL)
+		/* about to be stopped. */
+		return;
 
 	PRINTK("%s: bitmap writeback daemon woke up...\n", bmname(bitmap));
 	/* wait on bitmap page writebacks */