userns: Unbreak the unprivileged remount tests

commit db86da7c upstream.

A security fix in caused the way the unprivileged remount tests were
using user namespaces to break.  Tweak the way user namespaces are
being used so the test works again.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>