Fix broken fragment/mssfix with NCP

Fix broken fragment/mssfix with NCP

NCP negotiation replaces worst cast crypto overhead
with actual one in data channel frame. That frame
params are used by mssfix.

Fragment frame still contains worst case overhead.
Because of that TCP packets are fragmented, since
MSS value exceeds max fragment size.

Fix by replacing worst case crypto overhead with
actual one for fragment frame, as it is done for data
channel frame.

Trac #1140

Signed-off-by: Lev Stipakov <lstipakov@gmail.com>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Steffan Karger <steffan.karger@fox-it.com>
Message-Id: <1548101094-4449-1-git-send-email-lstipakov@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg18135.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>