repart: Ensure files end up owned by root in generated filesystems

repart: Ensure files end up owned by root in generated filesystems

By forking off a user namespace before running mkfs and ID mapping
the user running repart to root in the user namespace, we can make
sure that files in the generated filesystems are all owned by root
instead of the user running repart.

To make this work we have to make sure that all the files in the
root directory that's passed to the mkfs binary are owned by the
user running repart, so we have to drop the shortcut for only a
single root directory in partition_populate_directory().