this_cpu: Use this_cpu operations for SNMP statistics

SNMP statistic macros can be signficantly simplified.
This will also reduce code size if the arch supports these operations
in hardware.

Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Christoph Lameter <cl@linux-foundation.org>
Signed-off-by: Tejun Heo <tj@kernel.org>