rcu: restrict TREE_RCU to SMP builds with !PREEMPT

Because both TINY_RCU and TREE_PREEMPT_RCU have been in mainline for
several releases, it is time to restrict the use of TREE_RCU to SMP
non-preemptible systems.  This reduces testing/validation effort.  This
commit is a first step towards driving the selection of RCU implementation
directly off of the SMP and PREEMPT configuration parameters.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>