testutil: use filepath.WalkDir instead of filepath.Walk

WalkDir is more performant as it doesn't perform an os.Lstat on every visited
file or directory.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

testutil/daemon/daemon_linux.go

@@ -18,8 +18,7 @@ func cleanupNetworkNamespace(t testing.TB, d *Daemon) {
 	// daemon instance and has no chance of getting
 	// cleaned up when a new daemon is instantiated with a
 	// new exec root.
-	netnsPath := filepath.Join(d.execRoot, "netns")
-	filepath.Walk(netnsPath, func(path string, info os.FileInfo, err error) error {
+	filepath.WalkDir(filepath.Join(d.execRoot, "netns"), func(path string, _ os.DirEntry, _ error) error {
 		if err := unix.Unmount(path, unix.MNT_DETACH); err != nil && err != unix.EINVAL && err != unix.ENOENT {
 			t.Logf("[%s] unmount of %s failed: %v", d.id, path, err)
 		}