rcu: Create rcuo kthreads only for onlined CPUs

rcu: Create rcuo kthreads only for onlined CPUs

RCU currently uses for_each_possible_cpu() to spawn rcuo kthreads,
which can result in more rcuo kthreads than one would expect, for
example, derRichard reported 64 CPUs worth of rcuo kthreads on an
8-CPU image.  This commit therefore creates rcuo kthreads only for
those CPUs that actually come online.

This was reported by derRichard on the OFTC IRC network.

Reported-by: Richard Weinberger <[email protected]>
Signed-off-by: Paul E. McKenney <[email protected]>
Reviewed-by: Josh Triplett <[email protected]>
Tested-by: Paul Gortmaker <[email protected]>