mm, page_alloc: Add missing check for memory holes

Commit 13ad59df67f1 ("mm, page_alloc: avoid page_to_pfn() when merging
buddies") moved the check for memory holes out of page_is_buddy() and
had the callers do the check.

But this wasn't done correctly in one place which caused ia64 to crash
very early in boot.

Update to fix that and make ia64 boot again.

[ v2: Vlastimil pointed out we don't need to call page_to_pfn()
      since we already have the result of that in "buddy_pfn" ]

Fixes: 13ad59df67f1 ("avoid page_to_pfn() when merging buddies")
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page_alloc.c

@@ -873,7 +873,8 @@ done_merging:
 		higher_page = page + (combined_pfn - pfn);
 		buddy_pfn = __find_buddy_pfn(combined_pfn, order + 1);
 		higher_buddy = higher_page + (buddy_pfn - combined_pfn);
-		if (page_is_buddy(higher_page, higher_buddy, order + 1)) {
+		if (pfn_valid_within(buddy_pfn) &&
+		    page_is_buddy(higher_page, higher_buddy, order + 1)) {
 			list_add_tail(&page->lru,
 				&zone->free_area[order].free_list[migratetype]);
 			goto out;