vxlan: allow configuring port range

VXLAN bases source UDP port based on flow to help the
receiver to be able to load balance based on outer header flow.

This patch restricts the port range to the normal UDP local
ports, and allows overriding via configuration.

It also uses jhash of Ethernet header when looking at flows
with out know L3 header.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>