rbd: acquire header_rwsem just once in rbd_queue_workfn()

Currently header_rwsem is acquired twice: once in rbd_dev_parent_get()
when the image request is being created and then in rbd_queue_workfn()
to capture mapping_size and snapc.  Introduce rbd_img_capture_header()
and move image request allocation so that header_rwsem can be acquired
just once.

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