libceph: turn on CEPH_FEATURE_MSG_ADDR2

Now that the client can handle either address formatting, advertise to
the peer that we can support it.

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