genirq: Speedup show_interrupts()

genirq: Speedup show_interrupts()

Since commit 425a5072dcd1 ("genirq: Free irq_desc with rcu"),
show_interrupts() can be switched to rcu locking, which removes possible
contention on sparse_irq_lock.

The per_cpu count scan and print can be done without holding desc spinlock.

And there is no need to call kstat_irqs_cpu() and abuse irq_to_desc() while
holding rcu read lock, since desc and desc->kstat_irqs wont disappear or
change.

Signed-off-by: Eric Dumazet <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Cc: Eric Dumazet <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]