vxlan: Call udp_flow_src_port

In vxlan and OVS vport-vxlan call common function to get source port
for a UDP tunnel. Removed vxlan_src_port since the functionality is
now in udp_flow_src_port.

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