xfs: allow appending aio writes

XFS can easily support appending aio writes by ensuring we always allocate
blocks as unwritten extents when performing direct I/O writes and only
converting them to written extents at I/O completion.

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