mm: wrap __find_buddy_pfn() with a necessary buddy page validation

Whenever the buddy of a page is found from __find_buddy_pfn(),
page_is_buddy() should be used to check its validity.  Add a helper
function find_buddy_page_pfn() to find the buddy page and do the check
together.

[[email protected]: updates per David]
Link: https://lkml.kernel.org/r/[email protected]
Link: https://lore.kernel.org/linux-mm/CAHk-=wji_AmYygZMTsPMdJ7XksMt7kOur8oDfDdniBRMjm4VkQ@mail.gmail.com/
Link: https://lkml.kernel.org/r/[email protected]
Suggested-by: Linus Torvalds <[email protected]>
Signed-off-by: Zi Yan <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
Acked-by: David Hildenbrand <[email protected]>
Cc: Steven Rostedt (Google) <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Mike Rapoport <[email protected]>
Cc: Oscar Salvador <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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