Don't limit max incoming message size based on c2->frame

Don't limit max incoming message size based on c2->frame

"Be conservative in what you send, be liberal in what you accept"

When receiving packets, the real limitation of how much data we can accept
is the size of our internal buffers, not the maximum size we expect
incoming packets to have.

I ran into this while working on cipher negotiation, which will need
separate bookkeeping for the required internal buffer size, and the
link/tun MTU.  Basing this code on the buffer size instead of c2->frame
makes that easier.  A nice side-effect of this change is that it
simplifies the code.

This should also reduce the impact of using asymmetric tun/link MTU's,
such as in trac ticket #647.

Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1465388443-15484-2-git-send-email-steffan@karger.me>
URL: http://article.gmane.org/gmane.network.openvpn.devel/11850
Signed-off-by: Gert Doering <gert@greenie.muc.de>