Restore krb5_get_credentials caching for referral requests

Restore krb5_get_credentials caching for referral requests

The krb5_get_credentials() rewrite for IAKERB accidentally omitted the
final step of restoring the requested realm in the output credentials.
As a result, referral entries are not cached, and the caller sees the
actual realm in (*out_creds)->server instead of the referral realm as
before.  Fix this in complete() by swapping ctx->req_server into
ctx->reply_creds->server.

ticket: 6916
target_version: 1.9.2
tags: pullup

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@24945 dc483132-0cff-0310-8789-dd5450dbe970