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>
(cherry picked from commit 0c33930e3a90f3873b7c7b18ff70dec2894fce29)

Conflicts:
	src/mon/MDSMonitor.cc
Merge pull request #2990 from ceph/wip-10151-giant

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