ip_tunnel: Fix bugs that could crash kernel

ip_tunnel: Fix bugs that could crash kernel

Without this patch, OVS kernel module can delete itn->fb_tunnel_dev
one more time than necessary, which causes kernel crash.

On kernel 4.4.0-116-generic, the crash can be reproduced by running
the simple test provided below through check-kernel.

  make & make modules_install
  rmmod ip_gre gre ip_tunnel
  modprobe openvswitch
  make check-kernel TESTSUITEFLAGS=x
  dmesg

Simple test:

AT_SETUP([datapath - crash test])
OVS_CHECK_GRE()
ip link del gre0
OVS_TRAFFIC_VSWITCHD_START()
AT_CHECK([ovs-vsctl -- set bridge br0])
ADD_BR([br-underlay], [set bridge br-underlay])
AT_CHECK([ovs-ofctl add-flow br0 "actions=normal"])
AT_CHECK([ovs-ofctl add-flow br-underlay "actions=normal"])
ADD_NAMESPACES(at_ns0)
ADD_VETH(p0, at_ns0, br-underlay, "172.31.1.1/24")
AT_CHECK([ip addr add dev br-underlay "172.31.1.100/24"])
AT_CHECK([ip link set dev br-underlay up])
ADD_OVS_TUNNEL([gre], [br0], [at_gre0], [172.31.1.1], [10.1.1.100/24])
tcpdump -U -i br-underlay -w underlay.pcap &
sleep 1
OVS_TRAFFIC_VSWITCHD_STOP
AT_CLEANUP

Signed-off-by: Yifeng Sun <pkusunyifeng@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: William Tu <u9012063@gmail.com>