rcu: Protect rcu_check_gp_kthread_starvation() access to ->gp_flags

The rcu_check_gp_kthread_starvation() function can be invoked without
holding locks, so the access to the rcu_state structure's ->gp_flags
field must be protected with READ_ONCE().  This commit therefore adds
this protection.
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>

kernel/rcu/tree.c

@@ -1192,7 +1192,7 @@ static void rcu_check_gp_kthread_starvation(void)
 		pr_err("%s kthread starved for %ld jiffies! g%ld f%#x %s(%d) ->state=%#lx ->cpu=%d\n",
 		       rcu_state.name, j,
 		       (long)rcu_seq_current(&rcu_state.gp_seq),
-		       rcu_state.gp_flags,
+		       READ_ONCE(rcu_state.gp_flags),
 		       gp_state_getname(rcu_state.gp_state), rcu_state.gp_state,
 		       gpk ? gpk->state : ~0, gpk ? task_cpu(gpk) : -1);
 		if (gpk) {