disk: trim the device mapper name.

While reading the value of the file with `ioutil.ReadFile`,
the name is suffixed with a `\n`

Tested on Ubuntu 18.10

disk/disk_linux.go

@@ -433,7 +433,7 @@ func GetLabel(name string) string {
 	if err != nil {
 		return ""
 	} else {
-		return string(dmname)
+		return strings.TrimSpace(string(dmname))
 	}
 }