mon,cephfs: require confirmation when changing max_mds on unhealthy cluster

mon,cephfs: require confirmation when changing max_mds on unhealthy cluster

User must pass the confirmation flag (--yes-i-really-mean-it) to change
the value of CephFS setting variable "max_mds" when the Ceph cluster is
unhealthy.

This measure was decided upon to prevent users from changing "max_mds"
as a measure of troubleshotoing unhealthy cluster.

Fixes: https://tracker.ceph.com/issues/66301
Signed-off-by: Rishabh Dave <ridave@redhat.com>
(cherry picked from commit a55a75c57e7a42a1317e4d7fc86c1964b71137f0)

Conflicts:

src/mon/FSCommands.cc
- Method set_val() is present in this file on main branch but not on
  Reef branch