Always unblock parent when child setup fails, rather than just
exiting.

Also remove a duplicate call to setup_cgroup().  We'll want it
close to there for userns, but not right there - that's too late,
and could happen after container init has done something bad
without cgroup restrictions.
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>