mds-client: make the fake inos option unchangeable in runtime

mds-client: make the fake inos option unchangeable in runtime

If the flags is empty then in option.h in can_update_at_runtime()
it will return true. That means this opetion could be changed in
runtime, which is buggy. Because if this is false, ceph-fuse will
use its own fake inos instead of libcephfs'. If this is changed
during runtime, we will hit inos dosn't exist assert bugs.

Fixes: https://tracker.ceph.com/issues/54653
Signed-off-by: Xiubo Li <xiubli@redhat.com>