rbd: move OSD request allocation into object request state machines

Following submission, move initial OSD request allocation into object
request state machines.  Everything that has to do with OSD requests is
now handled inside the state machine, all __rbd_img_fill_request() has
left is initialization.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Dongsheng Yang <dongsheng.yang@easystack.cn>