fs_mgr_overlayfs: Polish fs_mgr_overlayfs_mount_fstab_entry() am: 94d1f3b4

Original change: https://googleplex-android-review.googlesource.com/c/platform/system/core/+/14660512

Change-Id: I640707a40f8567679c48d9a59b0e9f106995afff