Revert change of FATTR4_SEC_LABEL value

e04719ce7034 "Remove FATTR4_CHANGE_SEC_LABEL" shifted down the values of
three other attributes when it removed FATTR4_CHANGE_SEC_LABEL.

In particular, it redefined FATTR4_SEC_LABEL, which already has
implementations.

Let's leave these alone instead and just leave a hole.

Note that nobody appears to have ever actually implemented
FATTR4_CHANGE_SEC_LABEL, so it would be safe to fill in the hole later
if we want.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Tom Haynes <Thomas.Haynes@primarydata.com>