Get WWN for multipath block devices

Device Mapper devices have DM_WWN set in udev in place of
ID_WWN_WITH_VENDOR_EXTENSION. Reading DM_WWN allows us to see the WWN
for multipath fibre channel or iSCSI block devices.

Because DM_WWN contains the vendor extension, there is no corresponding
change to WWNNoExtension.

Signed-off-by: Zane Bitter <zbitter@redhat.com>

Author: zaneb

pkg/block/block_linux.go

@@ -208,6 +208,10 @@ func diskWWN(paths *linuxpath.Paths, disk string) string {
 	if wwn, ok := info["ID_WWN"]; ok {
 		return wwn
 	}
+	// Device Mapper devices get DM_WWN instead of ID_WWN_WITH_EXTENSION
+	if wwn, ok := info["DM_WWN"]; ok {
+		return wwn
+	}
 	return util.UNKNOWN
 }