test: add a test for fs set down on online cluster

Fixes: https://tracker.ceph.com/issues/66960
Signed-off-by: chungfengz <[email protected]>

Author: chungfengz-syno

qa/workunits/cephtool/test.sh

@@ -976,6 +976,11 @@ function test_mon_mds()
   ceph fs set $FS_NAME cluster_down true
   ceph fs set $FS_NAME cluster_down false
 
+  ceph fs set $FS_NAME max_mds 2
+  ceph fs get $FS_NAME | expect_true grep -P -q 'max_mds\t2'
+  ceph fs set $FS_NAME down false
+  ceph fs get $FS_NAME | expect_true grep -P -q 'max_mds\t2'
+
   ceph mds compat rm_incompat 4
   ceph mds compat rm_incompat 4