rcu: Remove initialized but unused rnp from check_slow_task()

This commit removes the variable rnp from check_slow_task(), which
is defined, assigned to, but not otherwise used.
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/tree_stall.h

@@ -237,14 +237,12 @@ struct rcu_stall_chk_rdr {
  */
 static bool check_slow_task(struct task_struct *t, void *arg)
 {
-	struct rcu_node *rnp;
 	struct rcu_stall_chk_rdr *rscrp = arg;
 
 	if (task_curr(t))
 		return false; // It is running, so decline to inspect it.
 	rscrp->nesting = t->rcu_read_lock_nesting;
 	rscrp->rs = t->rcu_read_unlock_special;
-	rnp = t->rcu_blocked_node;
 	rscrp->on_blkd_list = !list_empty(&t->rcu_node_entry);
 	return true;
 }