rcu: Move wakeup out from under rnp->lock

This patch removes a potential deadlock hazard by moving the
wake_up_process() in rcu_spawn_gp_kthread() out from under rnp->lock.

Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>