ceph: allow connecting to mds whose rank >= mdsmap::m_max_mds

mdsmap::m_max_mds is the expected count of active mds. It's not the
max rank of active mds. User can decrease mdsmap::m_max_mds, but does
not stop mds whose rank >= mdsmap::m_max_mds.

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