nspawn: make sure idmapped logic works if DDI contains only /usr/ tree

nspawn: make sure idmapped logic works if DDI contains only /usr/ tree

If we have a DDI that contains only a /usr/ tree (and which is thus
combined with a tmpfs for root on boot) we previously would try to apply
idmapping to the tmpfs, but not the /usr/ mount. That's broken of
course.

Fix this by applying it to both trees.