acpi/nfit: Fix race accessing memdev in nfit_get_smbios_id()

Possible race accessing memdev structures after dropping the
mutex. Dan Williams says this could race against another thread
that is doing:

 # echo "ACPI0012:00" > /sys/bus/acpi/drivers/nfit/unbind
Reported-by: Jane Chu <jane.chu@oracle.com>
Fixes: 23222f8f ("acpi, nfit: Add function to look up nvdimm...")
Signed-off-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

drivers/acpi/nfit/core.c

@@ -714,6 +714,7 @@ int nfit_get_smbios_id(u32 device_handle, u16 *flags)
 	struct acpi_nfit_memory_map *memdev;
 	struct acpi_nfit_desc *acpi_desc;
 	struct nfit_mem *nfit_mem;
+	u16 physical_id;
 
 	mutex_lock(&acpi_desc_lock);
 	list_for_each_entry(acpi_desc, &acpi_descs, list) {
@@ -721,10 +722,11 @@ int nfit_get_smbios_id(u32 device_handle, u16 *flags)
 		list_for_each_entry(nfit_mem, &acpi_desc->dimms, list) {
 			memdev = __to_nfit_memdev(nfit_mem);
 			if (memdev->device_handle == device_handle) {
+				*flags = memdev->flags;
+				physical_id = memdev->physical_id;
 				mutex_unlock(&acpi_desc->init_mutex);
 				mutex_unlock(&acpi_desc_lock);
-				*flags = memdev->flags;
-				return memdev->physical_id;
+				return physical_id;
 			}
 		}
 		mutex_unlock(&acpi_desc->init_mutex);