datapath: backport: geneve: avoid using stale geneve socket.

datapath: backport: geneve: avoid using stale geneve socket.

Upstream commit:
    commit fceb9c3e38252992bbf1a3028cc2f7b871211533
    Author: pravin shelar <pshelar@ovn.org>
    Date:   Fri Oct 28 09:59:16 2016 -0700

    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>
Signed-off-by: Pravin B Shelar <pshelar@ovn.org>
Acked-by: Joe Stringer <joe@ovn.org>