rcu:combine duplicate code, courtesy of CONFIG_PREEMPT_RCU
rcu:apply TINY_PREEMPT_RCU read-speedup to TREE_PREEMPT_RCU
rcu: performance fixes to TINY_PREEMPT_RCU callback checking

This commit tightens up checks in rcu_preempt_check_callbacks() to avoid
unnecessary special handling at rcu_read_unlock() time.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
rcu: fix sparse errors in rcutorture.c
rcu: Add tracing data to support queueing models