vxlan: factor out VXLAN-GPE next protocol

The values are shared between VXLAN-GPE and NSH. Originally probably by
coincidence but I notified both working groups about this last year and they
seem to keep the values in sync since then.

Hopefully they'll get a single IANA registry for the values, too. (I asked
them for that.)

Factor out the code to be shared by the NSH implementation.

NSH and MPLS values are added in this patch, too. For MPLS, the drafts
incorrectly assign only a single value, while we have two MPLS ethertypes.
I raised the problem with both groups. For now, I assume the value is for
unicast.

Signed-off-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>