qa/workunits/suites/fsx.sh: don't use zero range

Zero range is not supported by cephfs.

Fixes: #8542
Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #2045 from ceph/wip-8542

qa/workunits/suites/fsx.sh: don't use zero range

Reviewed-by: Greg Farnum <greg@inktank.com>