ip6_gre: add ip6 gre and gretap collect_md mode

Similar to gre, vxlan, geneve, ipip tunnels, allow ip6 gre and gretap
tunnels to operate in collect metadata mode.  bpf_skb_[gs]et_tunnel_key()
helpers can make use of it right away.  OVS can use it as well in the
future.

Signed-off-by: William Tu <u9012063@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>