auth/kerberos: Do a string comparison in kerberos_decode_pac() not a principal comparison

auth/kerberos: Do a string comparison in kerberos_decode_pac() not a principal comparison

This ensures that if an enterprise principal is used, we do the
comparison properly

This matters as in the enterprise case, which can be triggered by MIT
kinit -E, does not use canonicalization, and so the enterprise name,
with the @ in it, is in the logon name.

Otherwise, we get errors like:
 Name in PAC [TESTALLOWED@WIN2012R2] does not match principal name in ticket

BUG: https://bugzilla.samba.org/show_bug.cgi?id=11142

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Guenther Deschner <gd@samba.org>