percpu: fix generic definition of __this_cpu_add_and_return()

This patch adds missed "__" into function prefix.
Otherwise on all archectures (except x86) it expands to irq/preemtion-safe
variant: _this_cpu_generic_add_return(), which do extra irq-save/irq-restore.
Optimal generic implementation is __this_cpu_generic_add_return().

Signed-off-by: Konstantin Khlebnikov <khlebnikov@openvz.org>
Acked-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Tejun Heo <tj@kernel.org>