rcu: Update rcu_preempt_deferred_qs() comments for !PREEMPT kernels

In non-premptible kernels, tasks never do context switches within
RCU read-side critical sections.  Therefore, in such kernels, each
leaf rcu_node structure's ->blkd_tasks list will always be empty.
The comment on the non-preemptible version of rcu_preempt_deferred_qs()
confuses this point, so this commit therefore fixes it.

Signed-off-by: Zqiang <qiang1.zhang@intel.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>