rcu: init rcu_registry_lock after fork

We were unlocking this lock after fork, which is wrong since
only the thread that holds a mutex is allowed to unlock it.

Signed-off-by: Emilio G. Cota <cota@braap.org>
Message-Id: <1440375847-17603-9-git-send-email-cota@braap.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>