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

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 <[email protected]>
Reviewed-by: Dave Chinner <[email protected]>
Signed-off-by: Dave Chinner <[email protected]>