homed: when creating home directory as btrfs inside luks volume, place subvol inside it

homed: when creating home directory as btrfs inside luks volume, place subvol inside it

When managing a home directory as LUKS image we currently place a
directory at the top that contains the actual home directory (so that
the home directory of the user won't be cluttered by lost-found and
suchlike). On btrfs let's make that a subvol though. This is a good idea
so that possibly later on we can make use of this for automatic history
management.

Fixes: #15121