Fix is_ipv6 in case of tap interface.

While checking a packet on a TAP interface, is_ipv_X() in proto.c
insist that the ethertype must be OPENVPN_ETH_P_IPV4, even if
the protocol is IPv6. So the protocol never match, and, thus,
mssfix doesn't work for IPv6 on TAP interface. Fix that.

Signed-off-by: Dmitrij Tejblum <dt@yandex.ru>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1391873629-14388-1-git-send-email-dt@yandex.ru>
URL: http://article.gmane.org/gmane.network.openvpn.devel/8259
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit db037c20086587a609ef33127c15de080270f2cb)