cephfs: disallow removing root_squash via "fs authorize" cmd

cephfs: disallow removing root_squash via "fs authorize" cmd

Removing root_squasn from MDS auth caps through "fs authorize" command
should not be allowed as this command it not allowed to/meant for
removing caps.

Fixes: https://tracker.ceph.com/issues/65808
Signed-off-by: Rishabh Dave <ridave@redhat.com>
(cherry picked from commit c6e2c97c6e9cbf1e37c53d5d490d65091205928c)

Conflict:

- qa/tasks/cephfs/test_admin.py
  Test test_idem_unaffected_root_squash (which was fixed by this commit)
  was disabled on the main branch since it was buggy. But that wasn't
  the case with squid branch.