patch e4230030 in mainline.

This is a writeback-internal marker but we're propagating it all the way back
to userspace!.

Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>