libmount: when moving a mount point, all sub mount entries in utab should also be...

libmount: when moving a mount point, all sub mount entries in utab should also be updated

Given that /run/mount/utab stores paths, this file needs to be adjusted when a
mount tree is moved.

However the moved tree may contains sub mount points in which case their utab
entries (if any) need to also be translated.

This patch takes care of that.

As suggested in https://github.com/systemd/systemd/issues/15266, a better
approach might be to store mount IDs instead of paths since mount IDs remain
unchanged when mount trees are moved.

Fixes: #1659