net: bridge: fix MST static key usage

As Ido pointed out, the static key usage in MST is buggy and should use
inc/dec instead of enable/disable because we can have multiple bridges
with MST enabled which means a single bridge can disable MST for all.
Use static_branch_inc/dec to avoid that. When destroying a bridge decrement
the key if MST was enabled.

Fixes: ec7328b ("net: bridge: mst: Multiple Spanning Tree (MST) mode")
Reported-by: Ido Schimmel <idosch@nvidia.com>
Closes: https://lore.kernel.org/netdev/20251104120313.1306566-1-razor@blackwall.org/T/#m6888d87658f94ed1725433940f4f4ebb00b5a68b
Signed-off-by: Nikolay Aleksandrov <razor@blackwall.org>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20251105111919.1499702-3-razor@blackwall.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: NikAleksandrov

net/bridge/br_if.c

@@ -386,6 +386,7 @@ void br_dev_delete(struct net_device *dev, struct list_head *head)
 		del_nbp(p);
 	}
 
+	br_mst_uninit(br);
 	br_recalculate_neigh_suppress_enabled(br);
 
 	br_fdb_delete_by_port(br, NULL, 0, 1);

net/bridge/br_mst.c

@@ -22,6 +22,12 @@ bool br_mst_enabled(const struct net_device *dev)
 }
 EXPORT_SYMBOL_GPL(br_mst_enabled);
 
+void br_mst_uninit(struct net_bridge *br)
+{
+	if (br_opt_get(br, BROPT_MST_ENABLED))
+		static_branch_dec(&br_mst_used);
+}
+
 int br_mst_get_info(const struct net_device *dev, u16 msti, unsigned long *vids)
 {
 	const struct net_bridge_vlan_group *vg;
@@ -225,9 +231,9 @@ int br_mst_set_enabled(struct net_bridge *br, bool on,
 		return err;
 
 	if (on)
-		static_branch_enable(&br_mst_used);
+		static_branch_inc(&br_mst_used);
 	else
-		static_branch_disable(&br_mst_used);
+		static_branch_dec(&br_mst_used);
 
 	br_opt_toggle(br, BROPT_MST_ENABLED, on);
 	return 0;

net/bridge/br_private.h

@@ -1954,6 +1954,7 @@ int br_mst_fill_info(struct sk_buff *skb,
 		     const struct net_bridge_vlan_group *vg);
 int br_mst_process(struct net_bridge_port *p, const struct nlattr *mst_attr,
 		   struct netlink_ext_ack *extack);
+void br_mst_uninit(struct net_bridge *br);
 #else
 static inline bool br_mst_is_enabled(const struct net_bridge_port *p)
 {
@@ -1989,6 +1990,10 @@ static inline int br_mst_process(struct net_bridge_port *p,
 {
 	return -EOPNOTSUPP;
 }
+
+static inline void br_mst_uninit(struct net_bridge *br)
+{
+}
 #endif
 
 struct nf_br_ops {