fork: child should issue synchronize_rcu() for urcu-bp garbage collection

fork: child should issue synchronize_rcu() for urcu-bp garbage collection

After a fork(), the child process should execute synchronize_rcu() before any
new thread can be created. Failure to do so could lead to a deadlock in the
unlikely scenario where a thread ID appearing in the parent is reused in the
child before GC is performed.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>