rcu: Comment why callbacks migration can't wait for CPUHP_RCUTREE_PREP

The callbacks migration is performed through an explicit call from
the hotplug control CPU right after the death of the target CPU and
before proceeding with the CPUHP_ teardown functions.

This is unusual but necessary and yet uncommented. Summarize the reason
as explained in the changelog of:

	a58163d8ca (rcu: Migrate callbacks earlier in the CPU-offline timeline)

Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>

kernel/cpu.c

@@ -1372,7 +1372,14 @@ static int takedown_cpu(unsigned int cpu)
 	cpuhp_bp_sync_dead(cpu);
 
 	tick_cleanup_dead_cpu(cpu);
+
+	/*
+	 * Callbacks must be re-integrated right away to the RCU state machine.
+	 * Otherwise an RCU callback could block a further teardown function
+	 * waiting for its completion.
+	 */
 	rcutree_migrate_callbacks(cpu);
+
 	return 0;
 }