vmscan.c: fix invalid strict_strtoul() check in write_scan_unevictable_node()

write_scan_unevictable_node() checks the value req returned by
strict_strtoul() and returns 1 if req is 0.

However, when strict_strtoul() returns 0, it means successful conversion
of buf to unsigned long.

Due to this, the function was not proceeding to scan the zones for
unevictable pages even though we write a valid value to the
scan_unevictable_pages sys file.

Change this check slightly to check for invalid value in buf as well as 0
value stored in res after successful conversion via strict_strtoul.  In
both cases, we do not perform the scanning of this node's zones.

Signed-off-by: Kautuk Consul <consul.kautuk@gmail.com>
Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Johannes Weiner <jweiner@redhat.com>
Cc: Lee Schermerhorn <lee.schermerhorn@hp.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/vmscan.c

@@ -3520,8 +3520,8 @@ static ssize_t write_scan_unevictable_node(struct sys_device *dev,
 	unsigned long res;
 	unsigned long req = strict_strtoul(buf, 10, &res);
 
-	if (!req)
-		return 1;	/* zero is no-op */
+	if (req || !res)
+		return 1; /* Invalid input or zero is no-op */
 
 	for (zone = node_zones; zone - node_zones < MAX_NR_ZONES; ++zone) {
 		if (!populated_zone(zone))