rcu: Add rcu_barrier() statistics to debugfs tracing

This commit adds an rcubarrier file to RCU's debugfs statistical tracing
directory, providing diagnostic information on rcu_barrier().

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>