ip6_tunnel: allow redirecting ip6gre and ipxip6 packets to eth devices

Reset the mac_header pointer even when the tunnel transports only L3
data (in the ARPHRD_ETHER case, this is already done by eth_type_trans).
This prevents other parts of the stack from mistakenly accessing the
outer header after the packet has been decapsulated.

In practice, this allows to push an Ethernet header to ipip6, ip6ip6,
mplsip6 or ip6gre packets and redirect them to an Ethernet device:

  $ tc filter add dev ip6tnl0 ingress matchall       \
      action vlan push_eth dst_mac 00:00:5e:00:53:01 \
                           src_mac 00:00:5e:00:53:00 \
      action mirred egress redirect dev eth0

Without this patch, push_eth refuses to add an ethernet header because
the skb appears to already have a MAC header.

Signed-off-by: Guillaume Nault <gnault@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>