nspawn: mount /var/ after remount_idmap() when --volatile=state

nspawn: mount /var/ after remount_idmap() when --volatile=state

Previously, remount_idmap() failed as /var/ was already mounted, thus
remounting (strictly speaking, unmounting old root directory) failed
with -EBUSY.

As tmpfs /var/ is mounted with picked UID shift, it should not be
remounted with idmap, but needs to be mounted after the root directory
being remounted.

This makes '-U --volatile=state' work as expected.

(cherry picked from commit 2c2511aa734c507e04e06d273b474acacac9d486)