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>

include/linux/part_stat.h

@@ -54,7 +54,7 @@ static inline void part_stat_set_all(struct hd_struct *part, int value)
 	 part_stat_read(part, field[STAT_DISCARD]))
 
 #define __part_stat_add(part, field, addnd)				\
-	(part_stat_get(part, field) += (addnd))
+	__this_cpu_add((part)->dkstats->field, addnd)
 
 #define part_stat_add(part, field, addnd)	do {			\
 	__part_stat_add((part), field, addnd);				\