pktgen: fix crash at module unload

commit 7d3d43da (net: In unregister_netdevice_notifier unregister
the netdevices.) makes pktgen crashing at module unload.

[  296.820578] BUG: spinlock bad magic on CPU#6, rmmod/3267
[  296.820719]  lock: ffff880310c38000, .magic: ffff8803, .owner: <none>/-1, .owner_cpu: -1
[  296.820943] Pid: 3267, comm: rmmod Not tainted 3.4.0-rc5+ #254
[  296.821079] Call Trace:
[  296.821211]  [<ffffffff8168a715>] spin_dump+0x8a/0x8f
[  296.821345]  [<ffffffff8168a73b>] spin_bug+0x21/0x26
[  296.821507]  [<ffffffff812b4741>] do_raw_spin_lock+0x131/0x140
[  296.821648]  [<ffffffff8169188e>] _raw_spin_lock+0x1e/0x20
[  296.821786]  [<ffffffffa00cc0fd>] __pktgen_NN_threads+0x4d/0x140 [pktgen]
[  296.821928]  [<ffffffffa00ccf8d>] pktgen_device_event+0x10d/0x1e0 [pktgen]
[  296.822073]  [<ffffffff8154ed4f>] unregister_netdevice_notifier+0x7f/0x100
[  296.822216]  [<ffffffffa00d2a0b>] pg_cleanup+0x48/0x73 [pktgen]
[  296.822357]  [<ffffffff8109528e>] sys_delete_module+0x17e/0x2a0
[  296.822502]  [<ffffffff81699652>] system_call_fastpath+0x16/0x1b

Hold the pktgen_thread_lock while splicing pktgen_threads, and test
pktgen_exiting in pktgen_device_event() to make unload faster.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>