Comment 0 for bug 1824735

We currently keep a reference to the shiftfs mark mount's
shiftfs_super_info which was stashed in the superblock of the mark mount. The problem is that we only take a reference to the mount of the underlay, i.e. the filesystem that is *under* the shiftfs mark mount. This means when someone performs a shiftfs mark mount, then a shiftfs overlay mount and then immediately unmounts the shiftfs mark mount we muck with invalid memory since shiftfs_put_super might have already been called freeing that memory.