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>