CVE-2022-2031 s4:kdc: Fix canonicalisation of kadmin/changepw principal

Since this principal goes through the samba_kdc_fetch_server() path,
setting the canonicalisation flag would cause the principal to be
replaced with the sAMAccountName; this meant requests to
kadmin/changepw@REALM would result in a ticket to krbtgt@REALM. Now we
properly handle canonicalisation for the kadmin/changepw principal.

View with 'git show -b'.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15047

Pair-Programmed-With: Andreas Schneider <asn@samba.org>
Signed-off-by: Andreas Schneider <asn@samba.org>
Signed-off-by: Joseph Sutton <josephsutton@catalyst.net.nz>
Reviewed-by: Andreas Schneider <asn@samba.org>
