datapath: Avoid use-after-free error in dp_device_event().

datapath: Avoid use-after-free error in dp_device_event().

Commit f14d80834 "datapath: genl_notify() on port disappearances" frees the
vport before passing it to ovs_vport_cmd_build_info(), which reads the
freed data.

Without this commit, the following commands consistently trigger a kernel
BUG report on my test VM (which has slab debugging enabled) on 3 attempts:

    tunctl
    ovs-vsctl add-port br0 tap0
    tunctl -d tap0

With this commit, I consistently don't see the BUG, on a few hundred tries
in a tight loop.

The interesting log information is:

    device tap0 entered promiscuous mode
    device tap0 left promiscuous mode
    BUG: unable to handle kernel paging request at 6b6b6ba7
    IP: [<c88269ed>] get_vport_protected+0x8/0x52 [openvswitch_mod]
    *pde = 00000000
    Oops: 0000 [#1] SMP
    last sysfs file: /sys/devices/pci0000:00/0000:00:04.0/net/eth1/carrier
    Modules linked in: brcompat_mod openvswitch_mod

    Pid: 653, comm: tunctl Not tainted 2.6.37+ #25 /Bochs
    EIP: 0060:[<c88269ed>] EFLAGS: 00010246 CPU: 0
    EIP is at get_vport_protected+0x8/0x52 [openvswitch_mod]
    EAX: 6b6b6ba7 EBX: 00000000 ECX: 00000000 EDX: 00000000
    ESI: c6d98400 EDI: c5c32074 EBP: c6ff1de8 ESP: c6ff1de4
     DS: 007b ES: 007b FS: 00d8 GS: 0000 SS: 0068
    Process tunctl (pid: 653, ti=c6ff0000 task=c5c3a820 task.ti=c6ff0000)
    Stack:
     00000000 c6ff1df8 c8826b1d 6b6b6b6b c6d77ab0 c6ff1e14 c8826dc6 c7489160
     00000f40 c6d98400 c6d77ab0 00000000 c6ff1e3c c88286fa 00000000 00000000
     00000002 02ff1e34 00000000 c6d77ab0 c6df98a0 00000006 c6ff1e54 c8828aa2
    Call Trace:
     [<c8826b1d>] ? get_dpifindex+0x1b/0x31 [openvswitch_mod]
     [<c8826dc6>] ? ovs_vport_cmd_fill_info+0x40/0x183 [openvswitch_mod]
     [<c88286fa>] ? ovs_vport_cmd_build_info+0x3f/0x62 [openvswitch_mod]
     [<c8828aa2>] ? dp_device_event+0x56/0xb0 [openvswitch_mod]
     [<c1046c0d>] ? notifier_call_chain+0x6d/0x96
     [<c1046c52>] ? raw_notifier_call_chain+0xc/0xe
     [<c1254c10>] ? call_netdevice_notifiers+0x3c/0x43
     [<c1255029>] ? rollback_registered_many+0xd4/0x18b
     [<c1255146>] ? rollback_registered+0x23/0x28
     [<c1255199>] ? unregister_netdevice_queue+0x4e/0x6b
     [<c1227d2e>] ? tun_chr_close+0x3f/0x76

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Jesse Gross <jesse@nicira.com>