libceph: ADDR2 support for monmap

Switch the MonMap decoder to use the new decoding routine for
entity_addr_t's.

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