compat: Add ipv6 GRE and IPV6 Tunneling

compat: Add ipv6 GRE and IPV6 Tunneling

This patch backports upstream ipv6 GRE and tunneling into the OVS
OOT (Out of Tree) datapath drivers.  The primary reason for this
is to support the ERSPAN feature.

Because there is no previous history of ipv6 GRE and tunneling it is
not possible to exactly reproduce the history of all the files in
the patch.  The two newly added files - ip6_gre.c and ip6_tunnel.c -
are cut from whole cloth out of the upstream Linux 4.15 kernel and
then modified as necessary with compatibility layer fixups.
These two files already included parts of several other upstream
commits that also touched other upstream files.  As such, this
patch may incorporate parts or all of the following commits:

d350a82 net: erspan: create erspan metadata uapi header
c69de58 net: erspan: use bitfield instead of mask and offset
b423d13 net: erspan: fix use-after-free
214bb1c net: erspan: remove md NULL check
afb4c97 ip6_gre: fix potential memory leak in ip6erspan_rcv
50670b6 ip_gre: fix potential memory leak in erspan_rcv
a734321 ip6_gre: fix error path when ip6erspan_rcv failed
dd8d5b8 ip_gre: fix error path when erspan_rcv failed
293a199 ip6_gre: fix a pontential issue in ip6erspan_rcv
d91e8db5 net: erspan: reload pointer after pskb_may_pull
ae3e133 net: erspan: fix wrong return value
c05fad5 ip_gre: fix wrong return value of erspan_rcv
94d7d8f ip6_gre: add erspan v2 support
f551c91 net: erspan: introduce erspan v2 for ip_gre
1d7e2ed net: erspan: refactor existing erspan code
ef7baf5 ip6_gre: add ip6 erspan collect_md mode
5a963eb ip6_gre: Add ERSPAN native tunnel support
ceaa001 openvswitch: Add erspan tunnel support.
f192970 ip_gre: check packet length and mtu correctly in erspan tx
c84bed4 ip_gre: erspan device should keep dst
c122fda ip_gre: set tunnel hlen properly in erspan_tunnel_init
5513d08 ip_gre: check packet length and mtu correctly in erspan_xmit
935a974 ip_gre: get key from session_id correctly in erspan_rcv
1a66a83 gre: add collect_md mode to ERSPAN tunnel
84e54fe gre: introduce native tunnel support for ERSPAN

In cases where the listed commits also touched other source code
files then the patches are also listed separately within this
patch series.

Signed-off-by: Greg Rose <gvrose8192@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: William Tu <u9012063@gmail.com>