geneve: avoid using stale geneve socket.

This patch is similar to earlier vxlan patch.
Geneve device close operation frees geneve socket. This
operation can race with geneve-xmit function which
dereferences geneve socket. Following patch uses RCU
mechanism to avoid this situation.

Signed-off-by: Pravin B Shelar <pshelar@ovn.org>
Acked-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>