rcu: Use for_each_leaf_node_cpu() in online CPU iteration

Though mostly identical, ->qsmaskinit(A.K.A rcu_rnp_online_cpus()) is
sometimes more sparse than the corresponding part of cpu_possible_mask
for an RCU leaf node. So we use for_each_leaf_node_cpu() in
rcu_boost_kthread_setaffinity() instead to save some extra checks.

Signed-off-by: Boqun Feng <boqun.feng@gmail.com>