GetUsrDeviceNode: Use rootdev tool to resolve backing blockdev

The use of findmnt, and possibly falling back to dmsetup info is fragile
and does not work well with, e.g., btrfs.
Use the rootdev tool from the seismograph repo to resolve the backing
device.

kola/tests/util/update.go

@@ -16,7 +16,6 @@ package util
 
 import (
 	"fmt"
-	"strings"
 
 	"github.com/flatcar/mantle/kola/cluster"
 	"github.com/flatcar/mantle/platform"
@@ -31,14 +30,9 @@ func AssertBootedUsr(c cluster.TestCluster, m platform.Machine, usr string) {
 }
 
 func GetUsrDeviceNode(c cluster.TestCluster, m platform.Machine) string {
-	// find /usr dev (-f to see the first mount, not the sysext overlay mount)
-	usrdev := c.MustSSH(m, "findmnt -fno SOURCE /usr")
-
-	// XXX: if the /usr dev is /dev/mapper/usr, we're on a verity enabled
-	// image, so use dmsetup to find the real device.
-	if strings.TrimSpace(string(usrdev)) == "/dev/mapper/usr" {
-		usrdev = c.MustSSH(m, "echo -n /dev/$(sudo dmsetup info --noheadings -Co blkdevs_used usr)")
-	}
+	// The rootdev tool finds the backing block dev better than, e.g.,
+	// findmnt -fno SOURCE /usr and/or dmsetup info --noheadings -Co blkdevs_used usr
+	usrdev := c.MustSSH(m, "rootdev -s /usr")
 
 	return string(usrdev)
 }