ACPI / EC: fix NULL pointer dereference in acpi_ec_remove_query_handler()

Use list_for_each_entry_safe for iterating because handler may be freed
in the loop.

BUG: unable to handle kernel NULL pointer dereference at 000000000000002c
IP: [<ffffffff814d69c8>] acpi_ec_put_query_handler+0x7/0x1a
Call Trace:
 acpi_ec_remove_query_handler+0x87/0x97
 acpi_smbus_hc_remove+0x2a/0x44 [sbshc]
 acpi_device_remove+0x7b/0x9a
 __device_release_driver+0x7e/0x110
 driver_detach+0xb0/0xc0
 bus_remove_driver+0x54/0xe0
 driver_unregister+0x2b/0x60
 acpi_bus_unregister_driver+0x10/0x12
 acpi_smb_hc_driver_exit+0x10/0x12 [sbshc]
 SyS_delete_module+0x1b8/0x210
 system_call_fastpath+0x12/0x6a
Signed-off-by: Chris Bainbridge <chris.bainbridge@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/acpi/ec.c

@@ -861,7 +861,7 @@ void acpi_ec_remove_query_handler(struct acpi_ec *ec, u8 query_bit)
 		}
 	}
 	mutex_unlock(&ec->mutex);
-	list_for_each_entry(handler, &free_list, node)
+	list_for_each_entry_safe(handler, tmp, &free_list, node)
 		acpi_ec_put_query_handler(handler);
 }
 EXPORT_SYMBOL_GPL(acpi_ec_remove_query_handler);