rcu: Add comment to rcu_do_batch() identifying rcuoc code path

This commit adds a comment to help explain why the "else" clause of the
in_serving_softirq() "if" statement does not need to enforce a time limit.
The reason is that this "else" clause handles rcuoc kthreads that do not
block handlers for other softirq vectors.

Acked-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>

kernel/rcu/tree.c

@@ -2131,6 +2131,8 @@ static void rcu_do_batch(struct rcu_data *rdp)
 				break;
 			}
 		} else {
+			// In rcuoc context, so no worries about depriving
+			// other softirq vectors of CPU cycles.
 			local_bh_enable();
 			lockdep_assert_irqs_enabled();
 			cond_resched_tasks_rcu_qs();