vxlan: allow choosing destination port per vxlan

Allow configuring the default destination port on a per-device basis.
Adds new netlink paramater IFLA_VXLAN_PORT to allow setting destination
port when creating new vxlan.

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