include/linux/backing-dev.h: simplify wb_stat_sum

include/linux/backing-dev.h: simplify wb_stat_sum

wb_stat_sum() disables interrupts and calls __wb_stat_sum() which
eventually calls __percpu_counter_sum().  However, the percpu routine is
already irq-safe.  Simplify the code a bit by making wb_stat_sum()
directly call percpu_counter_sum_positive() and not disable interrupts.

Also remove the now-uneeded __wb_stat_sum() which was just a wrapper
over percpu_counter_sum_positive().

Link: http://lkml.kernel.org/r/1498230681-29103-1-git-send-email-nborisov@suse.com
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Jens Axboe <axboe@fb.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>