percpu_counter: percpu_counter_hotcpu_callback() cleanup

In commit ebd8fef3 ("percpu_counter: make percpu_counters_lock
irq-safe") we disabled irqs in percpu_counter_hotcpu_callback()

We can grab every counter spinlock without having to disable
irqs again.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Tejun Heo <tj@kernel.org>