pull-tar: Insist on foreign UID when copying

pull-tar: Insist on foreign UID when copying

If we're doing foreign UID range copying, we're going to be joining
a private user namespace before doing the copy. copy_tree() insists
on keeping all UIDs/GIDs the same when copying. Hence, all the
UIDs/GIDs of the files we're copying should be in the private UID
range, which means they need to be owned by the foreign UID range
and we always need to call mountfsd_mount_directory_fd(). So there's
no point in having a fallback path if the source directory is not
foreign UID range owned, we'd simply fail to copy it later. Hence,
insist on the source directory being foreign UID range owned.