homed: mark LUKS loopback file as "dirty" via xattr when in use

homed: mark LUKS loopback file as "dirty" via xattr when in use

Let's track the "dirty" state of a home directory backed by a LUKS
volume by setting a new xattr "home.home-dirty" on the backing file
whenever it is in use.

This allows us to later user this information to show a home directory
as "dirty". This is useful because we trim/allocate on log-out, and
if we don't do that a home directory will be larger than necessary. This
fact is something we should communicate to the admin.

The idea is that when an admin sees a user with a "dirty" home directory
they can ask them to log in, to clean up the dirty state, and thus trim
everything again.