rcu: Allow rcu_irq_enter_check_tick() from NMI

Eugenio managed to tickle #PF from NMI context which resulted in
hitting a WARN in RCU through irqentry_enter() ->
__rcu_irq_enter_check_tick().

However, this situation is perfectly sane and does not warrant an
WARN. The #PF will (necessarily) be atomic and not require messing
with the tick state, so early return is correct.  This commit
therefore removes the WARN.

Fixes: aaf2bc50 ("rcu: Abstract out rcu_irq_enter_check_tick() from rcu_nmi_enter()")
Reported-by: "Eugenio Pérez" <eupm90@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/tree.c

@@ -928,8 +928,8 @@ void __rcu_irq_enter_check_tick(void)
 {
 	struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
 
-	 // Enabling the tick is unsafe in NMI handlers.
-	if (WARN_ON_ONCE(in_nmi()))
+	// If we're here from NMI there's nothing to do.
+	if (in_nmi())
 		return;
 
 	RCU_LOCKDEP_WARN(rcu_dynticks_curr_cpu_in_eqs(),