Allow changing the file mode, UID, and GID through SETATTR.

Without allow_other, UID and GID are not allowed to be changed, because
it would not make sense.  Also, changing group or others' permissions
is not allowed either.

For read-only exports, +w cannot be set.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20210625142317.271673-5-mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>