rcu-tasks: Use context-switch hook for PREEMPT=y kernels

Currently, the PREEMPT=y version of rcu_note_context_switch() does not
invoke rcu_tasks_qs(), and we need it to in order to keep RCU Tasks
Trace's IPIs down to a dull roar.  This commit therefore enables this
hook.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>