xfs/194: fix the exception when run on 4k sector drives

xfs/194: fix the exception when run on 4k sector drives

The below command in "Test 4":

    xfs_io -c "pwrite -S 0x33 -b 512 `expr $blksize \* 2` 512"

will run failed on 4k sector drives. So I use sector size to
replace the hard-code 512.

And we won't run this case when $sector_size > $page_size / 8.

Signed-off-by: Zorro Lang <zlang@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>