rcu: Fix comment in rcu_organize_nocb_kthreads()

It used to be that the rcuo callback-offload kthreads were spawned
in rcu_organize_nocb_kthreads(), and the comment before the "for"
loop says as much.  However, this spawning has long since moved to
the CPU-hotplug code, so this commit fixes this comment.

Reported-by: Michalis Kokologiannakis <mixaskok@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>