include/linux/blkdev.h: use NULL instead of zero

Quite useless but it shuts up some warnings.
Signed-off-by: Michele Curti <michele.curti@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/blkdev.h

@@ -1564,7 +1564,7 @@ static inline int blk_rq_map_integrity_sg(struct request_queue *q,
 }
 static inline struct blk_integrity *bdev_get_integrity(struct block_device *b)
 {
-	return 0;
+	return NULL;
 }
 static inline struct blk_integrity *blk_get_integrity(struct gendisk *disk)
 {