ceph: avoid picking MDS that is not active

Ignore replication or auth frag data if it indicates an MDS that is not
active.  This can happen if the MDS shuts down and the client has stale
data about the namespace distribution across the MDS cluster.  If that's
the case, fall back to directing the request based on the auth cap (which
should always be accurate).

Signed-off-by: Sage Weil <sage@newdream.net>