aio-dio-regress: fix aio-dio-extend-stat on s390

aio-dio-regress: fix aio-dio-extend-stat on s390

Hi,

The following patch queries the sector size of the underlying device so
that we can size the write buffer appropriately.  Without this patch, we
try to do an O_DIRECT write of 1KB to a device that has a sector size of
4KB.  This returns EINVAL, of course.  I also noticed that the test was
not checking the right fields for the return code.  It was checking the
original iocb's nbytes field instead of event->res.  So, I fixed that.

Signed-off-by: Jeff Moyer <jmoyer@redhat.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>