ceph: touch existing cap when handling reply

Move cap to tail of session->s_caps list. So ceph_trim_caps() will
trim older caps first.

Signed-off-by: "Yan, Zheng" <zyan@redhat.com>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>