memcg: fix per_node_info cleanup

syzbot has triggered a NULL ptr dereference when allocation fault
injection enforces a failure and alloc_mem_cgroup_per_node_info
initializes memcg->nodeinfo only half way through.

But __mem_cgroup_free still tries to free all per-node data and
dereferences pn->lruvec_stat_cpu unconditioanlly even if the specific
per-node data hasn't been initialized.

The bug is quite unlikely to hit because small allocations do not fail
and we would need quite some numa nodes to make struct
mem_cgroup_per_node large enough to cross the costly order.

Link: http://lkml.kernel.org/r/[email protected]
Reported-by: [email protected]
Fixes: 00f3ca2 ("mm: memcontrol: per-lruvec stats infrastructure")
Signed-off-by: Michal Hocko <[email protected]>
Reviewed-by: Andrey Ryabinin <[email protected]>
Cc: Johannes Weiner <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/memcontrol.c

@@ -4108,6 +4108,9 @@ static void free_mem_cgroup_per_node_info(struct mem_cgroup *memcg, int node)
 {
 	struct mem_cgroup_per_node *pn = memcg->nodeinfo[node];
 
+	if (!pn)
+		return;
+
 	free_percpu(pn->lruvec_stat_cpu);
 	kfree(pn);
 }