percpu: preffity percpu header files

percpu macros are difficult to read.  It's partly because they're
fairly complex but also because they simply lack visual and
conventional consistency to an unusual degree.  The preceding patches
tried to organize macro definitions consistently by their roles.  This
patch makes the following cosmetic changes to improve overall
readability.

* Use consistent convention for multi-line macro definitions - "do {"
  or "({" are now put on their own lines and the line continuing '\'
  are all put on the same column.

* Temp variables used inside macro are consistently given "__" prefix.

* When a macro argument is passed to another macro or a function,
  putting extra parenthses around it doesn't help anything.  Don't put
  them.

* _this_cpu_generic_*() are renamed to this_cpu_generic_*() so that
  they're consistent with raw_cpu_generic_*().

* Reorganize raw_cpu_*() and this_cpu_*() definitions so that trivial
  wrappers are collected in one place after actual operation
  definitions.

* Other misc cleanups including reorganizing comments.

All changes in this patch are cosmetic and cause no functional
difference.

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