video: fix deferred io fsync()

If CONFIG_FB_DEFERRED_IO is set, but there are framebuffers
registered that does not make use of deferred io, then fsync()
on those framebuffers will result in a crash. Fix that.

This is needed for sh_mobile_lcdcfb since we always enable
deferred io at compile time but we may disable deferred io
for some types of hardware configurations.
Signed-off-by: Magnus Damm <damm@igel.co.jp>
Acked-by: Jaya Kumar <jayakumar.lkml@gmail.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>

drivers/video/fb_defio.c

@@ -60,6 +60,10 @@ int fb_deferred_io_fsync(struct file *file, struct dentry *dentry, int datasync)
 {
 	struct fb_info *info = file->private_data;
 
+	/* Skip if deferred io is complied-in but disabled on this fbdev */
+	if (!info->fbdefio)
+		return 0;
+
 	/* Kill off the delayed work */
 	cancel_rearming_delayed_work(&info->deferred_work);