Update snapshot mkdir to check for directory exist

When starting up a snapshot driver on subsequent runs, the
mkdir call will return an exist error, this can be safely
ignored.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

snapshot/overlay/overlay.go

@@ -49,7 +49,7 @@ func NewSnapshotter(root string) (snapshot.Snapshotter, error) {
 		return nil, err
 	}
 
-	if err := os.MkdirAll(filepath.Join(root, "snapshots"), 0700); err != nil {
+	if err := os.Mkdir(filepath.Join(root, "snapshots"), 0700); err != nil && !os.IsExist(err) {
 		return nil, err
 	}