rcu: Make use of rcu_preempt_has_tasks()

Given that there is now arcu_preempt_has_tasks() function that checks
to see if the ->blkd_tasks list is non-empty, this commit makes use of it.

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

diff --cc kernel/rcu/tree_plugin.h
Simple merge