xfs: always use unwritten extents for direct I/O writes

To allow aio writes beyond i_size we need to create unwritten extents for
newly allocated blocks, similar to how we already do inside i_size.

Instead of adding another special case we now use unwritten extents
unconditionally.  This also marks the end of directly allocation data
extents in all of XFS - we now always use either delalloc or unwritten
extents.

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