percpu_counter: add percpu_counter_sync()

percpu_counter's accuracy is related to its batch size.  For a
percpu_counter with a big batch, its deviation could be big, so when the
counter's batch is runtime changed to a smaller value for better accuracy,
there could also be requirment to reduce the big deviation.

So add a percpu-counter sync function to be run on each CPU.

Reported-by: kernel test robot <[email protected]>
Signed-off-by: Feng Tang <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Cc: Dennis Zhou <[email protected]>
Cc: Tejun Heo <[email protected]>
Cc: Christoph Lameter <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Qian Cai <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: Huang Ying <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Haiyang Zhang <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: "K. Y. Srinivasan" <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Tim Chen <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Linus Torvalds <[email protected]>

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