mm: restore node stat checking in /proc/sys/vm/stat_refresh

In v4.7 commit 52b6f46b ("mm: /proc/sys/vm/stat_refresh to force
vmstat update") introduced vmstat_refresh(), with its vmstat underflow
checking; then in v4.8 commit 75ef7184 ("mm, vmstat: add
infrastructure for per-node vmstats") split NR_VM_NODE_STAT_ITEMS out of
NR_VM_ZONE_STAT_ITEMS without updating vmstat_refresh(): so it has been
missing out much of the vmstat underflow checking ever since.

Reinstate it.

Thanks to Roman Gushchin <guro@fb.com> for tangentially pointing this out.

Link: https://lkml.kernel.org/r/alpine.LSU.2.11.2102251502240.13363@eggly.anvilsSigned-off-by: Hugh Dickins <hughd@google.com>
Cc: Roman Gushchin <guro@fb.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: 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

@@ -1875,6 +1875,14 @@ int vmstat_refresh(struct ctl_table *table, int write,
 		}
 	}
 #endif
+	for (i = 0; i < NR_VM_NODE_STAT_ITEMS; i++) {
+		val = atomic_long_read(&vm_node_stat[i]);
+		if (val < 0) {
+			pr_warn("%s: %s %ld\n",
+				__func__, node_stat_name(i), val);
+			err = -EINVAL;
+		}
+	}
 	if (err)
 		return err;
 	if (write)