net: bridge: mst: Restrict info size queries to bridge ports

Ensure that no bridge masters are ever considered for MST info
dumping. MST states are only supported on bridge ports, not bridge
masters - which br_mst_info_size relies on.

Fixes: 122c2948 ("net: bridge: mst: Support setting and reporting MST port states")
Signed-off-by: Tobias Waldekranz <tobias@waldekranz.com>
Link: https://lore.kernel.org/r/20220322133001.16181-1-tobias@waldekranz.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/bridge/br_netlink.c

@@ -119,7 +119,7 @@ static size_t br_get_link_af_size_filtered(const struct net_device *dev,
 	/* Each VLAN is returned in bridge_vlan_info along with flags */
 	vinfo_sz += num_vlan_infos * nla_total_size(sizeof(struct bridge_vlan_info));
 
-	if (vg && (filter_mask & RTEXT_FILTER_MST))
+	if (p && vg && (filter_mask & RTEXT_FILTER_MST))
 		vinfo_sz += br_mst_info_size(vg);
 
 	if (!(filter_mask & RTEXT_FILTER_CFM_STATUS))