Lock overlay.Driver.locker before creating a layer

It's _completely_ unclear to me why we need this
driver-specific locking when the whole layerStore
is locked before calling the driver; but if it makes
any sense at all, and Driver.Remove is taking the
lock, it seems that driver.create should take
the lock as well.

Note that additionalLayer.CreateAs is not
holding Driver.locker over its modifications;
this does not change that.

Signed-off-by: Miloslav Trmač <mitr@redhat.com>

drivers/overlay/overlay.go

@@ -913,6 +913,11 @@ func (d *Driver) create(id, parent string, opts *graphdriver.CreateOpts, disable
 		rootUID = int(st.UID())
 		rootGID = int(st.GID())
 	}
+
+	// --- Only access "dir" below.
+	d.locker.Lock(id)
+	defer d.locker.Unlock(id)
+
 	if err := idtools.MkdirAllAndChownNew(dir, 0700, idPair); err != nil {
 		return err
 	}