net_ads: fill ads->auth.realm from c->creds

net_ads: fill ads->auth.realm from c->creds

We get the realm we use for authentication needs to
the realm belonging to the username we use.

We derive the username from c->creds, so we need to
do the same for the realm.

Otherwise we try to authenticate as the wrong user.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15323

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>