libceph: advertise support for keepalive2

We are the client, but advertise keepalive2 anyway - for consistency,
if nothing else.  In the future the server might want to know whether
its clients support keepalive2.

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