ACPI battery: fix async boot oops

> BUG: unable to handle kernel NULL pointer dereference at (null)

What happens is that the battery module's init sections are being freed
before the async callback (which was marked __init) has run. This theory
is supported by the fact that the bad RIP value is a vmalloc address.

The immediate fix is to make this a non-init call.

(A better long-term fix is of course to wait with init-section unloading
until a module's async initcalls have been run, which would allow us to
discard this function which is still only run once, after all. Perhaps a
new async_initcall() function for the async/module API, if this is needed
for other modules in the future?)

Reported-by: Arkadiusz Miskiewicz <a.miskiewicz@gmail.com>
Signed-off-by: Vegard Nossum <vegard.nossum@gmail.com>
Tested-by: Alessandro Suardi <alessandro.suardi@gmail.com>
Tested-by: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: Len Brown <len.brown@intel.com>

drivers/acpi/battery.c

@@ -903,7 +903,7 @@ static struct acpi_driver acpi_battery_driver = {
 		},
 };
 
-static void __init acpi_battery_init_async(void *unused, async_cookie_t cookie)
+static void acpi_battery_init_async(void *unused, async_cookie_t cookie)
 {
 	if (acpi_disabled)
 		return;