rbd: drop original request earlier for existence check

The reference to the original request dropped at the end of
rbd_img_obj_exists_callback() corresponds to the reference taken
in rbd_img_obj_exists_submit() to account for the stat request
referring to it.  Move the put of that reference up right after
clearing that pointer to make its purpose more obvious.

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>