rbd: remove out_partial label in rbd_img_request_fill()

Commit 03507db6 ("rbd: fix buffer size for writes to images with
snapshots") moved the call to rbd_img_obj_request_add() up, making the
out_partial label bogus.  Remove it.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Reviewed-by: Alex Elder <elder@linaro.org>