s4:rpc_server: fix padding caclucation in dcesrv_auth_response()

s4:rpc_server: fix padding caclucation in dcesrv_auth_response()

This is simplified by using DCERPC_AUTH_PAD_LENGTH() and changes the behaviour
so that we will use no padding if the stub_length is already aligned
to DCERPC_AUTH_PAD_ALIGNMENT (16 bytes).

BUG: https://bugzilla.samba.org/show_bug.cgi?id=11061

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>