fbdev/deferred-io: Test screen_buffer for vmalloc'ed memory

Framebuffers in virtual memory are available via screen_buffer. Use
it instead of screen_base and avoid the type casting.
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240419083331.7761-5-tzimmermann@suse.de

drivers/video/fbdev/core/fb_defio.c

@@ -25,11 +25,11 @@
 
 static struct page *fb_deferred_io_page(struct fb_info *info, unsigned long offs)
 {
-	void *screen_base = (void __force *) info->screen_base;
+	const void *screen_buffer = info->screen_buffer;
 	struct page *page;
 
-	if (is_vmalloc_addr(screen_base + offs))
-		page = vmalloc_to_page(screen_base + offs);
+	if (is_vmalloc_addr(screen_buffer + offs))
+		page = vmalloc_to_page(screen_buffer + offs);
 	else
 		page = pfn_to_page((info->fix.smem_start + offs) >> PAGE_SHIFT);