mds: add root_squash mode in MDS auth caps

mds: add root_squash mode in MDS auth caps

Implement a root_squash mode in MDS auth caps to deny operations for
clients with uid=0 or gid=0 that need write access. It's mainly to
prevent operations such as accidental `sudo rm -rf /path`.

The root squash mode can be enforced in one of the following ways in
the MDS caps,
  'allow rw root_squash'
  (across file systems)
          or
  'allow rw fsname=a root_squash'
  (on a file system)
          or
  'allow rw fsname=a path=/vol/group/subvol00 root_squash'
  (on a file system path)

Fixes: https://tracker.ceph.com/issues/42451
Signed-off-by: Ramana Raja <rraja@redhat.com>