block: use __this_cpu_add() instead of access by smp_processor_id()

Most architectures have fast path to access percpu for current cpu.
The required preempt_disable() is provided by part_stat_lock().

[hch: rebased]

Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>