net: Combine GENEVE and VXLAN port notifiers into single functions

This patch merges the GENEVE and VXLAN code so that both functions pass
through a shared code path.  This way we can start the effort of using a
single function on the network device drivers to handle both of these
tunnel types.

Signed-off-by: Alexander Duyck <aduyck@mirantis.com>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>