tools/testing/nvdimm: Retain security state after overwrite

Overwrite retains the security state after completion of operation.  Fix
nfit_test to reflect this so that the kernel can test the behavior it is
more likely to see in practice.

Fixes: 926f7480 ("tools/testing/nvdimm: Add overwrite support for nfit_test")
Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>