Merge branch 'ip_tunnel-collect_md'

Alexei Starovoitov says:

====================
ip_tunnel: add collect_md mode to IPv4/IPv6 tunnels

Similar to geneve, vxlan, gre tunnels implement 'collect metadata' mode
in ipip, ipip6, ip6ip6 tunnels.
====================

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