nfit_test Make private definitions to command emulation

Move private definitions to command emulation.
These definitions were originally defined at include/uapi/linux/ndctl.h,
but they are used at only nfit_test emulation now.

Signed-off-by: Yasunori Goto <y-goto@jp.fujitsu.com>
Reviewed-by: Vishal Verma <vishal.l.verma@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>