mm, page_alloc: avoid page_to_pfn() when merging buddies

mm, page_alloc: avoid page_to_pfn() when merging buddies

On architectures that allow memory holes, page_is_buddy() has to perform
page_to_pfn() to check for the memory hole.  After the previous patch,
we have the pfn already available in __free_one_page(), which is the
only caller of page_is_buddy(), so move the check there and avoid
page_to_pfn().

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Vlastimil Babka <[email protected]>
Acked-by: Mel Gorman <[email protected]>
Cc: Joonsoo Kim <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: "Kirill A. Shutemov" <[email protected]>
Cc: Johannes Weiner <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>