rcu: gp_max is protected by root rcu_node's lock

Because gp_max is protected by root rcu_node's lock, this commit moves
the gp_max definition to the region of the rcu_node structure containing
fields protected by this lock.

Signed-off-by: Wei Yang <richard.weiyang@linux.alibaba.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>