net/mlx5: Bridge, set debugfs access right to root-only

As suggested during code review set the access rights for bridge 'fdb'
debugfs file to root-only.

Fixes: 791eb782 ("net/mlx5: Bridge, expose FDB state via debugfs")
Reported-by: Jakub Kicinski <kuba@kernel.org>
Link: https://lore.kernel.org/netdev/20230619120515.5045132a@kernel.org/Signed-off-by: Vlad Buslov <vladbu@nvidia.com>
Reviewed-by: Gal Pressman <gal@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>

drivers/net/ethernet/mellanox/mlx5/core/esw/bridge_debugfs.c

@@ -64,7 +64,7 @@ void mlx5_esw_bridge_debugfs_init(struct net_device *br_netdev, struct mlx5_esw_
 
 	bridge->debugfs_dir = debugfs_create_dir(br_netdev->name,
 						 bridge->br_offloads->debugfs_root);
-	debugfs_create_file("fdb", 0444, bridge->debugfs_dir, bridge,
+	debugfs_create_file("fdb", 0400, bridge->debugfs_dir, bridge,
 			    &mlx5_esw_bridge_debugfs_fops);
 }