dell-laptop: Fix allocating & freeing SMI buffer page

This commit fix kernel crash when probing for rfkill devices in dell-laptop
driver failed. Function free_page() was incorrectly used on struct page *
instead of virtual address of SMI buffer.

This commit also simplify allocating page for SMI buffer by using
__get_free_page() function instead of sequential call of functions
alloc_page() and page_address().
Signed-off-by: Pali Rohár <pali.rohar@gmail.com>
Acked-by: Michal Hocko <mhocko@suse.cz>
Cc: stable@vger.kernel.org
Signed-off-by: Darren Hart <dvhart@linux.intel.com>

drivers/platform/x86/dell-laptop.c

@@ -306,7 +306,6 @@ static const struct dmi_system_id dell_quirks[] __initconst = {
 };
 
 static struct calling_interface_buffer *buffer;
-static struct page *bufferpage;
 static DEFINE_MUTEX(buffer_mutex);
 
 static int hwswitch_state;
@@ -2068,12 +2067,11 @@ static int __init dell_init(void)
 	 * Allocate buffer below 4GB for SMI data--only 32-bit physical addr
 	 * is passed to SMI handler.
 	 */
-	bufferpage = alloc_page(GFP_KERNEL | GFP_DMA32);
-	if (!bufferpage) {
+	buffer = (void *)__get_free_page(GFP_KERNEL | GFP_DMA32);
+	if (!buffer) {
 		ret = -ENOMEM;
 		goto fail_buffer;
 	}
-	buffer = page_address(bufferpage);
 
 	ret = dell_setup_rfkill();
 
@@ -2135,7 +2133,7 @@ static int __init dell_init(void)
 fail_backlight:
 	dell_cleanup_rfkill();
 fail_rfkill:
-	free_page((unsigned long)bufferpage);
+	free_page((unsigned long)buffer);
 fail_buffer:
 	platform_device_del(platform_device);
 fail_platform_device2: