CVE-2020-25717: s3-auth: fix MIT Realm regression

CVE-2020-25717: s3-auth: fix MIT Realm regression

This looks like a regression introduced by the recent security fixes. This
commit should hopefully fixes it.

As a quick solution it might be possible to use the username map script based on
the example in https://bugzilla.samba.org/show_bug.cgi?id=14901#c0. We're not
sure this behaves identical, but it might work in the standalone server case.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14922

Reported-at: https://lists.samba.org/archive/samba/2021-November/238720.html

Pair-Programmed-With: Stefan Metzmacher <metze@samba.org>

Signed-off-by: Ralph Boehme <slow@samba.org>
Signed-off-by: Stefan Metzmacher <metze@samba.org>
(cherry picked from commit 1e61de8306604a0d3858342df8a1d2412d8d418b)