mount: move the MakePrivate to pkg/mount

The logic is unrelated to graphdriver.

Signed-off-by: Vincent Batts <vbatts@redhat.com>

mount/sharedsubtree_linux.go

@@ -0,0 +1,18 @@
+// +build linux
+
+package mount
+
+func MakePrivate(mountPoint string) error {
+	mounted, err := Mounted(mountPoint)
+	if err != nil {
+		return err
+	}
+
+	if !mounted {
+		if err := Mount(mountPoint, mountPoint, "none", "bind,rw"); err != nil {
+			return err
+		}
+	}
+
+	return ForceMount("", mountPoint, "none", "private")
+}