net: add NSH header structures and helpers

NSH (Network Service Header)[1] is a new protocol for service
function chaining, it can be handled as a L3 protocol like
IPv4 and IPv6, Eth + NSH + Inner packet or VxLAN-gpe + NSH +
Inner packet are two typical use cases.

This patch adds NSH header structures and helpers for NSH GSO
support and Open vSwitch NSH support.

[1] https://datatracker.ietf.org/doc/draft-ietf-sfc-nsh/

[Jiri: added nsh_hdr() helper and renamed the header struct to "struct
nshhdr" to match the usual pattern. Removed packet type defines, these are
now shared with VXLAN-GPE.]
Signed-off-by: Yi Yang <yi.y.yang@intel.com>
Signed-off-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>