udp: Add function to make source port for UDP tunnels

This patch adds udp_flow_src_port function which is intended to be
a common function that UDP tunnel implementations call to set the source
port. The source port is chosen so that a hash over the outer headers
(IP addresses and UDP ports) acts as suitable hash for the flow of the
encapsulated packet. In this manner, UDP encapsulation works with RSS
and ECMP based wrt the inner flow.

Signed-off-by: Tom Herbert <therbert@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>