[giant backport] 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 cd179befcfbe8ee7cd6a2cae6962fc6333ac6a1b)

Conflicts:
	src/mon/MDSMonitor.cc