s4:librpc/rpc: fix padding caclucation in ncacn_push_request_sign()

s4:librpc/rpc: fix padding caclucation in ncacn_push_request_sign()

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>
(cherry picked from commit 114c52e73ed9e0adeac8ad1bc1dc014f3c10f4d6)