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 <[email protected]>

Author: chungfengz-syno

src/mon/FSCommands.cc

@@ -598,6 +598,11 @@ int FileSystemCommandHandler::set_val(Monitor *mon, FSMap& fsmap, MonOpRequestRe
 
       ss << fsp->get_mds_map().get_fs_name();
 
+      if (!is_down && fsp->get_mds_map().get_max_mds() > 0) {
+        ss << " is already online";
+        return 0;
+      }
+
       modify_filesystem(fsmap, fsv,
           [is_down](auto&& fs)
       {