libceph, rbd: add error handling for osd_req_op_cls_init()

Add proper error handling for osd_req_op_cls_init() to replace
BUG_ON statement when failing from memory allocation.

Signed-off-by: Chengguang Xu <cgxu519@gmx.com>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>