nfit, address-range-scrub: introduce nfit_spa->ars_state

In preparation for re-working the ARS implementation to better handle
short vs long ARS runs, introduce nfit_spa->ars_state. For now this just
replaces the nfit_spa->ars_required bit-field/flag, but going forward it
will be used to track ARS completion and make short vs long ARS
requests.

Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>