rbd: move OSD request submission into object request state machines

Start eliminating asymmetry where the initial OSD request is allocated
and submitted from outside the state machine, making error handling and
restarts harder than they could be.  This commit deals with submission,
a commit that deals with allocation will follow.

Note that this commit adds parent chain recursion on the submission
side:

  rbd_img_request_submit
    rbd_obj_handle_request
      __rbd_obj_handle_request
        rbd_obj_handle_read
          rbd_obj_handle_write_guard
            rbd_obj_read_from_parent
              rbd_img_request_submit

This will be fixed in the next commit.

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