block: rbd: use NULL instead of 0

The local variables such as 'bio_list', and 'pages' are pointers;
thus, use NULL instead of 0 to fix the following sparse warnings.

drivers/block/rbd.c:2166:32: warning: Using plain integer as NULL pointer
drivers/block/rbd.c:2168:31: warning: Using plain integer as NULL pointer
Signed-off-by: NJingoo Han <jg1.han@samsung.com>
Reviewed-by: NSage Weil <sage@inktank.com>

drivers/block/rbd.c

@@ -2163,9 +2163,9 @@ static int rbd_img_request_fill(struct rbd_img_request *img_request,
 	struct rbd_obj_request *obj_request = NULL;
 	struct rbd_obj_request *next_obj_request;
 	bool write_request = img_request_write_test(img_request);
-	struct bio *bio_list = 0;
+	struct bio *bio_list = NULL;
 	unsigned int bio_offset = 0;
-	struct page **pages = 0;
+	struct page **pages = NULL;
 	u64 img_offset;
 	u64 resid;
 	u16 opcode;