call_rcu threads should clear their PAUSED flag when they unpause

call_rcu threads should clear their PAUSED flag when they unpause

And call_rcu_after_fork_parent should spin-wait on this.

Otherwise a second fork in the parent will see the PAUSED flags
already set and call_rcu_before_fork will not correctly wait for the
call_rcu threads to quiesce on this second occasion.

Fixes #786

Signed-off-by: Keir Fraser <keir@cohodata.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>