mm/page_alloc.c: initialize num_movable in move_freepages()

If move_freepages_block() returns 0 because !zone_spans_pfn(),
*num_movable can hold the value from the stack because it does not get
initialized in move_freepages().

Move the initialization to move_freepages_block() to guarantee the value
actually makes sense.

This currently doesn't affect its only caller where num_movable != NULL,
so no bug fix, but just more robust.

Link: http://lkml.kernel.org/r/alpine.DEB.2.21.1810051355490.212229@chino.kir.corp.google.comSigned-off-by: David Rientjes <rientjes@google.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Greg Thelen <gthelen@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page_alloc.c

@@ -2021,10 +2021,6 @@ static int move_freepages(struct zone *zone,
 	          pfn_valid(page_to_pfn(end_page)) &&
 	          page_zone(start_page) != page_zone(end_page));
 #endif
-
-	if (num_movable)
-		*num_movable = 0;
-
 	for (page = start_page; page <= end_page;) {
 		if (!pfn_valid_within(page_to_pfn(page))) {
 			page++;
@@ -2064,6 +2060,9 @@ int move_freepages_block(struct zone *zone, struct page *page,
 	unsigned long start_pfn, end_pfn;
 	struct page *start_page, *end_page;
 
+	if (num_movable)
+		*num_movable = 0;
+
 	start_pfn = page_to_pfn(page);
 	start_pfn = start_pfn & ~(pageblock_nr_pages-1);
 	start_page = pfn_to_page(start_pfn);