vxlan: Restructure vxlan socket apis.

Restructure vxlan-socket management APIs so that it can be
shared between vxlan and ovs modules.
This patch does not change any functionality.

Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
v6-v7:
 - get rid of zero refcnt vs from hashtable.
Signed-off-by: David S. Miller <davem@davemloft.net>