mon: fix `fs set down` to adjust max_mds only when cluster is not down

mon: fix `fs set down` to adjust max_mds only when cluster is not down

Applying `fs set down false` on an up cluster will set the cluster's max_mds to 1, regardless of the cluster's current max_mds.
`fs set down false` should only change max_mds when the cluster was set to down. Otherwise, the cluster should remain unchanged.

Fixes: https://tracker.ceph.com/issues/66960
Signed-off-by: chungfengz <chungfengz@synology.com>
(cherry picked from commit 6bfce64f7e97726d27b872077d6b9addab87ab64)

 Conflicts:
	src/mon/FSCommands.cc

Minor conflict to use `fs->mds_map...` instead of `fsp->get_mds_map()...`
and the use of modify_filesystem() helper.