rcu: Make synchronize_rcu() fastpath update only boot-CPU counters

Large systems can have hundreds of rcu_node structures, and updating
counters in each of them might slow down booting.  This commit therefore
updates only the counters in those rcu_node structures corresponding
to the boot CPU, up to and including the root rcu_node structure.

The counters for the remaining rcu_node structures are updated by the
rcu_scheduler_starting() function, which executes just before the first
non-boot kthread is spawned.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>