xfstests: 226: have xfs_io use bigger buffers

xfstests: 226: have xfs_io use bigger buffers

By default xfs_io uses a buffer size of 4096 bytes.  On test 226,
the result is that the test runs much slower (at least an order
of magnitude) than it needs to.

Add a flag to the "pwrite" command sent to xfs_io so it uses
larger buffers, thereby speeding things up considerably.

Signed-off-by: Alex Elder <aelder@sgi.com>
Reviewed-by: Eric Sandeen <sandeen@sandeen.net>