overlay/031: fix test with inodes index enabled

overlay/031: fix test with inodes index enabled

When overlayfs is configured with CONFIG_OVERLAY_FS_INDEX=y,
workdir from previous overlay mount cannot be reused in a new
overlay mount that uses a different upper dir.

Fix the test to use a different workdir when mounting with a
different upper dir.

This change has no effect on older kernels and overlay
configured without CONFIG_OVERLAY_FS_INDEX.

Cc: zhangyi (F) <yi.zhang@huawei.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: zhangyi (F) <yi.zhang@huawei.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>