mm: fix dubious code in __count_immobile_pages()

When pfn_valid_within() failed 'iter' was incremented twice.
Signed-off-by: Namhyung Kim <namhyung@gmail.com>
Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page_alloc.c

@@ -5376,10 +5376,9 @@ __count_immobile_pages(struct zone *zone, struct page *page, int count)
 	for (found = 0, iter = 0; iter < pageblock_nr_pages; iter++) {
 		unsigned long check = pfn + iter;
 
-		if (!pfn_valid_within(check)) {
-			iter++;
+		if (!pfn_valid_within(check))
 			continue;
-		}
+
 		page = pfn_to_page(check);
 		if (!page_count(page)) {
 			if (PageBuddy(page))