rcu: Bring diagnostic read of rcu_state.gp_flags into alignment

This commit adds READ_ONCE() to a lockless diagnostic read from
rcu_state.gp_flags to avoid giving the compiler any chance whatsoever
of confusing the diagnostic state printed.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>

kernel/rcu/tree_stall.h

@@ -579,7 +579,7 @@ static void rcu_check_gp_kthread_expired_fqs_timer(void)
 		pr_err("%s kthread timer wakeup didn't happen for %ld jiffies! g%ld f%#x %s(%d) ->state=%#x\n",
 		       rcu_state.name, (jiffies - jiffies_fqs),
 		       (long)rcu_seq_current(&rcu_state.gp_seq),
-		       data_race(rcu_state.gp_flags),
+		       data_race(READ_ONCE(rcu_state.gp_flags)), // Diagnostic read
 		       gp_state_getname(RCU_GP_WAIT_FQS), RCU_GP_WAIT_FQS,
 		       data_race(READ_ONCE(gpk->__state)));
 		pr_err("\tPossible timer handling issue on cpu=%d timer-softirq=%u\n",