genirq: Use rcu in kstat_irqs_usr()

Jeremy Dorfman identified mutex contention when multiple threads
parse /proc/stat concurrently.

Since commit 425a5072 ("genirq: Free irq_desc with rcu"),
kstat_irqs_usr() can be switched to rcu locking, which removes this mutex
contention.

show_interrupts() case will be handled in a separate patch.

Reported-by: Jeremy Dorfman <jdorfman@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Willem de Bruijn <willemb@google.com>
Link: https://lkml.kernel.org/r/20180618125612.155057-1-edumazet@google.com