XFS will have to look at iocb->private to fix aio+dio.  No other filesystem
is using the blockdev_direct_IO* end_io callback.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>