iwlwifi: avoid a panic when unloading the module with RF Kill

When HW RF kill switch is set to kill the radio, our NIC issues an
interrupt after we stop the APM module. When we unload the module,
the driver disables and cleans the interrupts before stopping the
APM. So we have a real interrupt (inta not zero) pending.
When this interrupts pops up the tasklet has already been killed
and we crash.

Here is a logical description of the flow:

disable and clean interrupts
synchronize interrupts
kill the tasklet

stop the APM <<== creates an RF kill interrupt

free_irq <<== somehow our ISR is called here and we crash

Here is the panic message:

[  201.313636] BUG: unable to handle kernel paging request at ffff8800911b7150
[  201.314541] IP: [<ffffffff8106d652>] tasklet_action+0x62/0x130
[  201.315149] PGD 1c06063 PUD db37f067 PMD db408067 PTE 80000000911b7160
[  201.316456] Oops: 0000 [#1] SMP DEBUG_PAGEALLOC
[  201.317324] CPU 1
[  201.317495] Modules linked in: arc4 iwlwifi(-) mac80211 cfg80211 netconsole configfs binfmt_misc i915 drm_kms_helper drm uvcvideo i2c_algo_bit videodev dell_laptop dcdbas intel_agp dell_wmi intel_ips psmouse intel_gtt v4l2_compat_ioctl32 asix usbnet mii serio_raw video sparse_keymap firewire_ohci sdhci_pci sdhci firewire_core e1000e crc_itu_t [last unloaded: configfs]
[  201.323839]
[  201.324015] Pid: 2061, comm: modprobe Not tainted 3.1.0-rc9-wl #4 Dell Inc. Latitude E6410/0667CC
[  201.324736] RIP: 0010:[<ffffffff8106d652>]  [<ffffffff8106d652>] tasklet_action+0x62/0x130
[  201.325128] RSP: 0018:ffff88011bc43ea0  EFLAGS: 00010286
[  201.325338] RAX: ffff88008ae70000 RBX: ffff8800911b7150 RCX: ffff88008ae70028
[  201.325555] RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff88008ae70000
[  201.325775] RBP: ffff88011bc43ec0 R08: 0000000000000000 R09: 0000000000000000
[  201.325994] R10: 0000000000000002 R11: 0000000000000001 R12: 0000000000000001
[  201.326212] R13: 0000000000000006 R14: 0000000000000100 R15: ffff88008e259fd8
[  201.326431] FS:  00007f4b90ea9700(0000) GS:ffff88011bc40000(0000) knlGS:0000000000000000
[  201.326657] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
[  201.326864] CR2: ffff8800911b7150 CR3: 000000008fd6d000 CR4: 00000000000006e0
[  201.327083] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[  201.327302] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
[  201.327521] Process modprobe (pid: 2061, threadinfo ffff88008e258000, task ffff88008ae70000)
[  201.327747] Stack:
[  201.330494]  0000000000000046 0000000000000030 0000000000000001 0000000000000006
[  201.333870]  ffff88011bc43f30 ffffffff8106cd8a ffffffff811e1016 ffff88011bc43f08
[  201.337186]  0000000100000046 ffff88008e259fd8 0000000a10be2160 0000000000000006
[  201.340458] Call Trace:
[  201.342994]  <IRQ>
[  201.345656]  [<ffffffff8106cd8a>] __do_softirq+0xca/0x250
[  201.348185]  [<ffffffff811e1016>] ? pde_put+0x76/0x90
[  201.350730]  [<ffffffff8131aeae>] ? do_raw_spin_unlock+0x5e/0xb0
[  201.353261]  [<ffffffff811e1016>] ? pde_put+0x76/0x90
[  201.355776]  [<ffffffff8163ccfc>] call_softirq+0x1c/0x30
[  201.358287]  [<ffffffff8101531d>] do_softirq+0x9d/0xd0
[  201.360823]  [<ffffffff8106cb05>] irq_exit+0xd5/0xf0
[  201.363330]  [<ffffffff8163d5d6>] do_IRQ+0x66/0xe0
[  201.365819]  [<ffffffff81632673>] common_interrupt+0x73/0x73
[  201.368257]  <EOI>

Cc: <stable@kernel.org> 3.1+
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>