ipv6: gre: fix wrong skb->protocol in WCCP

When using GRE redirection in WCCP, it sets the wrong skb->protocol,
that is, ETH_P_IP instead of ETH_P_IPV6 for the encapuslated traffic.

Fixes: c12b395a ("gre: Support GRE over IPv6")
Cc: Dmitry Kozlov <xeb@mail.ru>
Signed-off-by: Yuri Chislov <yuri.chislov@gmail.com>
Tested-by: Yuri Chislov <yuri.chislov@gmail.com>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>