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

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

Upstream commit:
    commit c6fcc4fc5f8b592600c7409e769ab68da0fb1eca
    Author: pravin shelar <pshelar@ovn.org>
    Date:   Fri Oct 28 09:59:15 2016 -0700

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