net: bridge: fix under estimation in br_get_linkxstats_size()

Commit de179966 ("net: bridge: add STP xstats")
added an additional nla_reserve_64bit() in br_fill_linkxstats(),
but forgot to update br_get_linkxstats_size() accordingly.

This can trigger the following in rtnl_stats_get()

	WARN_ON(err == -EMSGSIZE);

Fixes: de179966 ("net: bridge: add STP xstats")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Vivien Didelot <vivien.didelot@gmail.com>
Cc: Nikolay Aleksandrov <nikolay@nvidia.com>
Acked-by: Nikolay Aleksandrov <nikolay@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/bridge/br_netlink.c

@@ -1667,6 +1667,7 @@ static size_t br_get_linkxstats_size(const struct net_device *dev, int attr)
 
 	return numvls * nla_total_size(sizeof(struct bridge_vlan_xstats)) +
 	       nla_total_size_64bit(sizeof(struct br_mcast_stats)) +
+	       (p ? nla_total_size_64bit(sizeof(p->stp_xstats)) : 0) +
 	       nla_total_size(0);
 }