xfs: remove suport for filesystems without unwritten extent flag

The option to enable unwritten extents was made default in 2003,
removed from mkfs in 2007, and cannot be disabled in v5.  We also
rely on it for a lot of common functionality, so filesystems without
it will run a completely untested and buggy code path.  Enabling the
support also is a simple bit flip using xfs_db, so legacy file
systems can still be brought forward.

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