tun: use is_tun_p2p more consistently

tun: use is_tun_p2p more consistently

Using "tun" as the variable name for the return of
is_tun_p2p is probably a historical accident. But
it has actual consequences in that the other code
often seems to assume that it does less checks
than it actually does.

Use "tun_p2p" as the variable name and remove checks
that are not required. Also use is_tun_p2p in more
places.

Change-Id: Ice8b95f953c3f7e71657a78ea12b02a08c60aa67
Signed-off-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by: Arne Schwabe <arne-openvpn@rfc2549.org>
Message-Id: <20240906162514.78671-1-frank@lichtenheld.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg29091.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>