generic/018: use xfs_io and larger buffers for writes

generic/018: use xfs_io and larger buffers for writes

The test currently uses 'dd' directly for writing to files; instead
we should be using the xfs_io pwrite command.

Also, when we have a configuration that does not do delayed
allocation (e.g. dax), there is no guarantee that the files will be
allocated in the pattern expected, so do all the writes from a
single buffer so the kernel can allocate extents in the manner the
test expects as much as possible.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>