Merge branch 'ipv6-gre-collect_md'

William Tu says:

====================
add ip6 gre and gretap collect_md mode

Similar to gre, vxlan, geneve, ipip tunnels, allow ip6gretap tunnels to
operate in collect metadata mode.  The first patch adds the support to
ip6_gre.c. The second patch enables unsetting the csum for ipv6 tunnel,
when using bpf_skb_[gs]et_tunnel_key() helpers.  Finally, the last patch
adds the ip6 gre and gretap tunnel test cases to BPF sample code.

The corresponding iproute2 patch:
https://marc.info/?l=linux-netdev&m=151216943128087&w=2
====================

Signed-off-by: David S. Miller <davem@davemloft.net>