percpu_counter: use this_cpu_ptr() instead of per_cpu_ptr()

this_cpu_ptr() avoids an array lookup and can use the percpu offset of the
local cpu directly.

Signed-off-by: Christoph Lameter <cl@linux.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>