I don't think only user mounts is the right filter.
We have support for lazy unmounts so it is possible to handle
Technically all we need to do is transform d_mounted from a counter
to a hlist_head and thread yet another list through struct vfs_mount
to track this.
I need to think about the semantics a little more before I have a good
feel of what makes sense. In particular do we want a full
recursive lazy unmount or do we want to handle submounts in a different
This also intersects in interesting ways with dcache pruning, and