mon: fix `fs ls` on peons

This was incorrectly using pending_mdsmap instead
of mdsmap.  We didn't notice in test because of
single-mon configurations.

Fixes: #10288
Backport: giant

Signed-off-by: John Spray <john.spray@redhat.com>
(cherry picked from commit 5559e6aea9e9374ecdac0351777dfd6f5f5d1e67)