rcu: Add failure tracing to rcutorture

Trace the rcutorture RCU accesses and dump the trace buffer when the
first failure is detected.

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>