ip6_tunnel: Update skb->protocol to ETH_P_IPV6 in ip6_tnl_xmit()

This patch updates skb->protocol to ETH_P_IPV6 in ip6_tnl_xmit() when an
IPv6 header is installed to a socket buffer.

This is not a cosmetic change.  Without updating this value, GSO packets
transmitted through an ipip6 tunnel have the protocol of ETH_P_IP and
skb_mac_gso_segment() will attempt to call gso_segment() for IPv4,
which results in the packets being dropped.

Fixes: b8921ca8 ("ip4ip6: Support for GSO/GRO")
Signed-off-by: Eli Cooper <elicooper@gmx.com>
Signed-off-by: David S. Miller <davem@davemloft.net>