Correctly calculate packet id size when epoch packet format is in use

Correctly calculate packet id size when epoch packet format is in use

The code assumed that always when tls mode (without CFB/OFB) is in use
that the packet size is 4 bytes. With epoch packet format is incorrect
as that uses 64 bit.

Even thought packet_id_long_form has a the same size (8 byte) it is not
the same header format (32 bit time + 32 bit IV) as the epoch
format (16 bit epoch + 48 IV). Use a simple sizeof(uint64_t) to
avoid suggesting that it might be the same.

Github: closes OpenVPN/openvpn#1074

Change-Id: I5b862eabe032eb4d2229ff5b2f4f6af7406f6f4e
Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Antonio Quartulli <antonio@mandelbit.com>
Gerrit URL: https://gerrit.openvpn.net/c/openvpn/+/1829
Message-Id: <20260729064158.15731-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg37983.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>