Drivers: hv: vmbus: unregister panic notifier on module unload

Commit 96c1d058 ("Drivers: hv: vmbus: Add
support for VMBus panic notifier handler") introduced
atomic_notifier_chain_register() call on module load. We also need to call
atomic_notifier_chain_unregister() on module unload as otherwise the following
crash is observed when we bring hv_vmbus back:

[   39.788877] BUG: unable to handle kernel paging request at ffffffffa00078a8
[   39.788877] IP: [<ffffffff8109d63f>] notifier_call_chain+0x3f/0x80
...
[   39.788877] Call Trace:
[   39.788877]  [<ffffffff8109de7d>] __atomic_notifier_call_chain+0x5d/0x90
...
[   39.788877]  [<ffffffff8109d788>] ? atomic_notifier_chain_register+0x38/0x70
[   39.788877]  [<ffffffff8109d767>] ? atomic_notifier_chain_register+0x17/0x70
[   39.788877]  [<ffffffffa002814f>] hv_acpi_init+0x14f/0x1000 [hv_vmbus]
[   39.788877]  [<ffffffff81002144>] do_one_initcall+0xd4/0x210
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/hv/vmbus_drv.c

@@ -1108,6 +1108,10 @@ static void __exit vmbus_exit(void)
 	hv_synic_clockevents_cleanup();
 	hv_remove_vmbus_irq();
 	vmbus_free_channels();
+	if (ms_hyperv.features & HV_FEATURE_GUEST_CRASH_MSR_AVAILABLE) {
+		atomic_notifier_chain_unregister(&panic_notifier_list,
+						 &hyperv_panic_block);
+	}
 	bus_unregister(&hv_bus);
 	hv_cleanup();
 	for_each_online_cpu(cpu)