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>