rbd: fix a couple warnings

gcc isn't quite smart enough and generates these warnings:

drivers/block/rbd.c: In function 'rbd_img_request_fill':
drivers/block/rbd.c:1266:22: warning: 'bio_list' may be used uninitialized in this function [-Wmaybe-uninitialized]
drivers/block/rbd.c:2186:14: note: 'bio_list' was declared here
drivers/block/rbd.c:2247:10: warning: 'pages' may be used uninitialized in this function [-Wmaybe-uninitialized]

even though they are initialized for their respective code paths.
Signed-off-by: Sage 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;
+	struct bio *bio_list = 0;
 	unsigned int bio_offset = 0;
-	struct page **pages;
+	struct page **pages = 0;
 	u64 img_offset;
 	u64 resid;
 	u16 opcode;