tcp_metrics: add missing barriers on delete

When removing an item from RCU protected list, we must prevent
store-tearing, using rcu_assign_pointer() or WRITE_ONCE().

Fixes: 04f721c6 ("tcp_metrics: Rewrite tcp_metrics_flush_all")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

net/ipv4/tcp_metrics.c

@@ -908,7 +908,7 @@ static void tcp_metrics_flush_all(struct net *net)
 			match = net ? net_eq(tm_net(tm), net) :
 				!refcount_read(&tm_net(tm)->ns.count);
 			if (match) {
-				*pp = tm->tcpm_next;
+				rcu_assign_pointer(*pp, tm->tcpm_next);
 				kfree_rcu(tm, rcu_head);
 			} else {
 				pp = &tm->tcpm_next;
@@ -949,7 +949,7 @@ static int tcp_metrics_nl_cmd_del(struct sk_buff *skb, struct genl_info *info)
 		if (addr_same(&tm->tcpm_daddr, &daddr) &&
 		    (!src || addr_same(&tm->tcpm_saddr, &saddr)) &&
 		    net_eq(tm_net(tm), net)) {
-			*pp = tm->tcpm_next;
+			rcu_assign_pointer(*pp, tm->tcpm_next);
 			kfree_rcu(tm, rcu_head);
 			found = true;
 		} else {