rcu: merge TREE_PREEPT_RCU blocked_tasks[] lists

Combine the current TREE_PREEMPT_RCU ->blocked_tasks[] lists in the
rcu_node structure into a single ->blkd_tasks list with ->gp_tasks
and ->exp_tasks tail pointers.  This is in preparation for RCU priority
boosting, which will add a third dimension to the combinatorial explosion
in the ->blocked_tasks[] case, but simply a third pointer in the new
->blkd_tasks case.

Also update documentation to reflect blocked_tasks[] merge

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

kernel/rcutree.c

@@ -1901,10 +1901,7 @@ static void __init rcu_init_one(struct rcu_state *rsp,
 					      j / rsp->levelspread[i - 1];
 			}
 			rnp->level = i;
-			INIT_LIST_HEAD(&rnp->blocked_tasks[0]);
-			INIT_LIST_HEAD(&rnp->blocked_tasks[1]);
-			INIT_LIST_HEAD(&rnp->blocked_tasks[2]);
-			INIT_LIST_HEAD(&rnp->blocked_tasks[3]);
+			INIT_LIST_HEAD(&rnp->blkd_tasks);
 		}
 	}