percpu: Replace __get_cpu_var with this_cpu_ptr

__this_cpu_ptr is being phased out.  Use raw_cpu_ptr instead which was
introduced in 3.15-rc1.  One case of using __get_cpu_var in the
get_cpu_var macro for address calculation was remaining in
include/linux/percpu.h.

tj: Updated patch description.

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