Fix mssfix and frame calculation in CBC mode

Fix mssfix and frame calculation in CBC mode

This commit fixes the MSS calculation in CBC mode. This fix has two parts:

- Added rounding to a multiple of block size during calculation of overhead
- In CBC mode the packet ID is part of the plaintext (or payload) rather
  than part of the header (like for AEAD), adjust the functions to
  correctly reflect this.

OCC link calculation is not affected since it ignores rounding of CBC
block size completely.

Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20220101162532.2251835-3-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg23494.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>