tools/testing/nvdimm: fix allocation range for mock flush hint tables

Commit 480b6837 "nvdimm: fix PHYS_PFN/PFN_PHYS mixup" identified
that we were passing an invalid address to devm_nvdimm_ioremap(). With
that fixed it exposed a bug in the memory reservation size for flush
hint tables.  Since we map a full page we need to mock a full page of
memory to back the flush hint table entries.

Cc: Oliver O'Halloran <oohall@gmail.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>