vxlan: Add vxlan recv demux.

Once we have ovs-vxlan functionality, one UDP port can be assigned
to kernel-vxlan or ovs-vxlan port.  Therefore following patch adds
vxlan demux functionality, so that vxlan or ovs module can
register for particular port.

Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>