vxlan: avoid using stale vxlan socket.

When vxlan device is closed vxlan socket is freed. This
operation can race with vxlan-xmit function which
dereferences vxlan socket. Following patch uses RCU
mechanism to avoid this situation.

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