nvdimm(4): Export NVDIMM health flags via sysctl

nvdimm(4): Export NVDIMM health flags via sysctl

The ACPI NFIT specification defines a set of "NVDIMM State Flags". These
flags are already reported by `acpidump -t', but this change makes them
available on a per-device basis, in a format that is more easily parsed.

To simplify this, introduce acpi_nfit_get_memory_maps_by_dimm(), which
locates the (ACPI_NFIT_MEMORY_MAP)s associated with a given
(nfit_handle_t).

Reviewed by:	mav, cem
Tested by:	mav, rpokala (version for stable/12)
MFC after:	3 days
Sponsored by:	Panasas
Differential Revision:	https://reviews.freebsd.org/D28700

(cherry picked from commit bdde49b7c7232c6936525f84ffb90856a7cd8e74)