libceph: add maybe_move_osd_to_lru() and switch to it

Abstract out __move_osd_to_lru() logic from __unregister_request() and
__unregister_linger_request().

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