vxlan: compute source port in network byte order

vxlan: compute source port in network byte order

Rather than computing source port and returning it in host order
then swapping later, go ahead and compute it in network order to
start with. Cleaner and less error prone.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>