drivers: enhance check for mount fs type

when checking that a mount has a specific file system type, also
validate that it is different than its parent directory.

This helps when using virtiofs as the underlying file system since the
check used for fuse-overlayfs would fail since they both use FUSE.

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

drivers/driver_linux.go

@@ -1,3 +1,4 @@
+//go:build linux
 // +build linux
 
 package graphdriver
@@ -168,5 +169,16 @@ func Mounted(fsType FsMagic, mountPath string) (bool, error) {
 	if err := unix.Statfs(mountPath, &buf); err != nil {
 		return false, err
 	}
-	return FsMagic(buf.Type) == fsType, nil
+	if FsMagic(buf.Type) != fsType {
+		return false, nil
+	}
+	// it is already the root
+	if mountPath == "/" {
+		return true, nil
+	}
+
+	if err := unix.Statfs(filepath.Dir(mountPath), &buf); err != nil {
+		return true, err
+	}
+	return FsMagic(buf.Type) != fsType, nil
 }