rbd: embed image request in blk-mq pdu

Avoid making allocations for !IMG_REQ_CHILD image requests.  Only
IMG_REQ_CHILD image requests need to be freed now.

Move the initial request checks to rbd_queue_rq().  Unfortunately we
can't fill the image request and kick the state machine directly from
rbd_queue_rq() because ->queue_rq() isn't allowed to block.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>