tunnel: Add layer 2 IPv6 GRE encapsulation support.

tunnel: Add layer 2 IPv6 GRE encapsulation support.

The patch adds ip6gre support. Tunnel type 'ip6gre' with packet_type=
legacy_l2 is a layer 2 GRE tunnel over IPv6, carrying inner ethernet packets
and encap with GRE header with outer IPv6 header.  Encapsulation of layer 3
packet over IPv6 GRE, ip6gre, is not supported yet.  I tested it by running:
  # make check-kernel TESTSUITEFLAGS='-k ip6gre'
under kernel 5.2 and for userspace:
  # make check TESTSUITEFLAGS='-k ip6gre'

Tested-by: Greg Rose <gvrose8192@gmail.com>
Tested-at: https://travis-ci.org/gvrose8192/ovs-experimental/builds/552977116
Reviewed-by: Greg Rose <gvrose8192@gmail.com>
Reviewed-by: Eli Britstein <elibr@mellanox.com>
Signed-off-by: William Tu <u9012063@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>