drm/radeon: fix use after free in ATRM bios reading code.

commit de47a9cd upstream.

Fixes:
https://bugs.freedesktop.org/show_bug.cgi?id=45503

Reported-and-Debugged-by: mlambda@gmail.com
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/gpu/drm/radeon/radeon_atpx_handler.c

@@ -59,8 +59,9 @@ static int radeon_atrm_call(acpi_handle atrm_handle, uint8_t *bios,
 
 	obj = (union acpi_object *)buffer.pointer;
 	memcpy(bios+offset, obj->buffer.pointer, obj->buffer.length);
+	len = obj->buffer.length;
 	kfree(buffer.pointer);
-	return obj->buffer.length;
+	return len;
 }
 
 bool radeon_atrm_supported(struct pci_dev *pdev)