overlay: create the merged path only if it does not exist

follow-up for ccb70a7

more information here: containers#1827 (comment)

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

drivers/overlay/overlay.go

@@ -1666,9 +1666,12 @@ func (d *Driver) get(id string, disableShifting bool, options graphdriver.MountO
 	}
 
 	mergedDir := path.Join(dir, "merged")
-	// Create the driver merged dir
-	if err := idtools.MkdirAs(mergedDir, 0o700, rootUID, rootGID); err != nil && !os.IsExist(err) {
-		return "", err
+	// attempt to create the merged dir only if it doesn't exist.
+	if _, err := os.Stat(mergedDir); err != nil {
+		// Create the driver merged dir
+		if err := idtools.MkdirAs(mergedDir, 0o700, rootUID, rootGID); err != nil && !os.IsExist(err) {
+			return "", err
+		}
 	}
 	if count := d.ctr.Increment(mergedDir); count > 1 {
 		return mergedDir, nil