acpi, nfit: rework NVDIMM leaf method detection

acpi, nfit: rework NVDIMM leaf method detection

Some BIOSen do not handle 0-byte transfer lengths for the _LSR and _LSW
(label storage read/write) methods. This causes Linux to fallback to the
deprecated _DSM path, or otherwise disable label support.

Introduce acpi_nvdimm_has_method() to detect whether a method is
available rather than calling the method, require _LSI and _LSR to be
paired, and require read support before enabling write support.

Cc: <stable@vger.kernel.org>
Fixes: a1d2fbe9c6e0 ("acpi, nfit: add support for the _LS...")
Suggested-by: Erik Schmauss <erik.schmauss@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>