acpi/nfit: Block function zero DSMs

In preparation for using function number 0 as an error value, prevent it
from being considered a valid function value by acpi_nfit_ctl().

Cc: <stable@vger.kernel.org>
Cc: stuart hayes <stuart.w.hayes@gmail.com>
Fixes: e02fb726 ("nfit: add Microsoft NVDIMM DSM command set...")
Reported-by: Jeff Moyer <jmoyer@redhat.com>
Reviewed-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

drivers/acpi/nfit/core.c

@@ -1867,6 +1867,13 @@ static int acpi_nfit_add_dimm(struct acpi_nfit_desc *acpi_desc,
 		return 0;
 	}
 
+	/*
+	 * Function 0 is the command interrogation function, don't
+	 * export it to potential userspace use, and enable it to be
+	 * used as an error value in acpi_nfit_ctl().
+	 */
+	dsm_mask &= ~1UL;
+
 	guid = to_nfit_uuid(nfit_mem->family);
 	for_each_set_bit(i, &dsm_mask, BITS_PER_LONG)
 		if (acpi_check_dsm(adev_dimm->handle, guid,