[PATCH] md: define and use safe_put_page for md

md sometimes call put_page on NULL pointers (treating it like kfree).  This is
not safe, so define and use a 'safe_put_page' which checks for NULL.

Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

drivers/md/bitmap.c

@@ -626,8 +626,7 @@ static void bitmap_file_unmap(struct bitmap *bitmap)
 	kfree(map);
 	kfree(attr);
 
-	if (sb_page)
-		put_page(sb_page);
+	safe_put_page(sb_page);
 }
 
 static void bitmap_stop_daemon(struct bitmap *bitmap);