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>

init/Kconfig

@@ -340,6 +340,7 @@ choice
 
 config TREE_RCU
 	bool "Tree-based hierarchical RCU"
+	depends on !PREEMPT && SMP
 	help
 	  This option selects the RCU implementation that is
 	  designed for very large SMP system with hundreds or