percpu: Generic support for this_cpu_add, sub, dec, inc_return

Introduce generic support for this_cpu_add_return etc.

The fallback is to realize these operations with simpler __this_cpu_ops.

tj: - Reformatted __cpu_size_call_return2() to make it more consistent
      with its neighbors.
    - Dropped unnecessary temp variable ret__ from
      __this_cpu_generic_add_return().

Reviewed-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Acked-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Tejun Heo <tj@kernel.org>