RDMA/mlx5: Block FDB rules when not in switchdev mode

Allow creating FDB steering rules only when in switchdev mode.

The only software model where a userspace application can manipulate
FDB entries is when it manages the eswitch. This is only possible in
switchdev mode where we expose a single RDMA device with representors
for all the vports that are connected to the eswitch.

Fixes: 52438be ("RDMA/mlx5: Allow inserting a steering rule to the FDB")
Link: https://lore.kernel.org/r/e928ae7c58d07f104716a2a8d730963d1bd01204.1623052923.git.leonro@nvidia.com
Reviewed-by: Maor Gottlieb <maorg@nvidia.com>
Signed-off-by: Mark Bloch <mbloch@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

drivers/infiniband/hw/mlx5/fs.c

@@ -2134,6 +2134,12 @@ static int UVERBS_HANDLER(MLX5_IB_METHOD_FLOW_MATCHER_CREATE)(
 	if (err)
 		goto end;
 
+	if (obj->ns_type == MLX5_FLOW_NAMESPACE_FDB &&
+	    mlx5_eswitch_mode(dev->mdev) != MLX5_ESWITCH_OFFLOADS) {
+		err = -EINVAL;
+		goto end;
+	}
+
 	uobj->object = obj;
 	obj->mdev = dev->mdev;
 	atomic_set(&obj->usecnt, 0);