mm/vmstat: fix divide error at __fragmentation_index

When order is -1 or too big, *1UL << order* will be 0, which will cause
a divide error.  Although it seems that all callers of
__fragmentation_index() will only do so with a valid order, the patch
can make it more robust.

Should prevent reoccurrences of
https://bugzilla.kernel.org/show_bug.cgi?id=196555

Link: http://lkml.kernel.org/r/1501751520-2598-1-git-send-email-wen.yang99@zte.com.cn
Signed-off-by: Wen Yang <wen.yang99@zte.com.cn>
Reviewed-by: Jiang Biao <jiang.biao2@zte.com.cn>
Suggested-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/vmstat.c

@@ -870,6 +870,9 @@ static int __fragmentation_index(unsigned int order, struct contig_page_info *in
 {
 	unsigned long requested = 1UL << order;
 
+	if (WARN_ON_ONCE(order >= MAX_ORDER))
+		return 0;
+
 	if (!info->free_blocks_total)
 		return 0;