direct-io: add flag to allow aio writes beyond i_size

Some filesystems can handle direct I/O writes beyond i_size safely,
so allow them to opt into receiving them.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 09f553c..f7faefc 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2527,6 +2527,9 @@
 
 	/* filesystem does not support filling holes */
 	DIO_SKIP_HOLES	= 0x02,
+
+	/* filesystem can handle aio writes beyond i_size */
+	DIO_ASYNC_EXTEND = 0x04,
 };
 
 void dio_end_io(struct bio *bio, int error);