Patch from William Lee Irwin III <wli@holomorphy.com>

Okay, this one looks ugly because we're missing some of the definitions
available with which to convert to inline functions (esp. struct page).
A lot of these introduce temporaries and sort of hope names won't clash,
which might be important to whoever cares about -Wshadow.

 - node_end_pfn() evaluates nid twice
 - local_mapnr() evaluates kvaddr twice
 - kern_addr_valid() evaluates kaddr twice
 - pfn_to_page() evaluates pfn multiple times
 - page_to_pfn() evaluates page thrice
 - pfn_valid() doesn't parenthesize its argument