Add support for rootless overlay mounts

This patch sets the userxattr field when run in rootless mode.

Fixes: https://github.com/containers/buildah/issues/3503

Patch came from @flouthoc  Aditya Rajan <arajan@redhat.com>

[NO TESTS NEEDED] Since this can not be tested in CI/CD system yet.

Signed-off-by: Daniel J Walsh <dwalsh@redhat.com>

pkg/overlay/overlay.go

@@ -144,6 +144,7 @@ func mountHelper(contentDir, source, dest string, _, _ int, graphOptions []strin
 			return mount, nil
 		}
 		/* If a mount_program is not specified, fallback to try mount native overlay.  */
+		overlayOptions = fmt.Sprintf("%s,userxattr", overlayOptions)
 	}
 
 	mount.Source = mergeDir