rcu: Fix invoke_rcu_callbacks() comment

Despite what the comment says, it is only softirqs that are disabled,
not interrupts.  This commit therefore fixes the comment.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcu/tree.c

@@ -2601,7 +2601,7 @@ static void rcu_process_callbacks(struct softirq_action *unused)
  * Schedule RCU callback invocation.  If the specified type of RCU
  * does not support RCU priority boosting, just do a direct call,
  * otherwise wake up the per-CPU kernel kthread.  Note that because we
- * are running on the current CPU with interrupts disabled, the
+ * are running on the current CPU with softirqs disabled, the
  * rcu_cpu_kthread_task cannot disappear out from under us.
  */
 static void invoke_rcu_callbacks(struct rcu_state *rsp, struct rcu_data *rdp)