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: Jingoo Han <jg1.han@samsung.com>
Reviewed-by: Sage Weil <sage@inktank.com>