rcu: Export RCU-tasks APIs to GPL modules

This commit exports the RCU-tasks synchronous APIs,
synchronize_rcu_tasks() and rcu_barrier_tasks(), to
GPL-licensed kernel modules.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>