mds/MDSAuthCaps: allow updating root_squash in entity's MDS cap

mds/MDSAuthCaps: allow updating root_squash in entity's MDS cap

Command "ceph fs authorize" can update fsname, path and perm/spec in a
MDS cap but it can't update "root_squash" in it. Add code to allow this.

Specifically, "MDSAuthCaps::merge_one_cap_grant()" doesn't check if
the value for "root_squash" needs an update while attempting to merge
the new MDS cap into the list of MDS caps already held by an entity. It
should do so to ensure correct behaviour.

Fixes: https://tracker.ceph.com/issues/64417
Signed-off-by: Rishabh Dave <ridave@redhat.com>