libceph: use sizeof_footer() more

Don't open-code sizeof_footer() in read_partial_message() and
ceph_msg_revoke().  Also, after switching to sizeof_footer(), it's now
possible to use con_out_kvec_add() in prepare_write_message_footer().

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Alex Elder <elder@linaro.org>