is_ipv_X: add support for parsing IP header inside a 802.1q frame

is_ipv_X: add support for parsing IP header inside a 802.1q frame

Extend is_ipv_X() routine by properly parsing 802.1q frame rather than
dropping them.

This change is required in order to allow OpenVPN to accept VLAN tagged
frames, which otherwise would be dropped when trying to access the inner
IP header.

While at it, slightly fix the function style.

Signed-off-by: Fabian Knittel <fabian.knittel@lettink.de>
Signed-off-by: Antonio Quartulli <a@unstable.cc>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20191009143422.9419-6-a@unstable.cc>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg18916.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>