mon: fix MDS health status from peons

The health data was there, but we were attempting
to enumerate MDS GIDs from pending_mdsmap (empty on
peons) instead of mdsmap (populated from paxos updates)

Fixes: #10151
Backport: giant

Signed-off-by: John Spray <john.spray@redhat.com>
Merge pull request #2996 from ceph/wip-10151

mon: fix MDS health status from peons

Reviewed-by: Greg Farnum <gfarnum@redhat.com>