ixgbe: fix panic when using macvlan with l2-fwd-offload enabled

Fix NULL pointer dereference in the case where a macvlan interface is
brought up while the PF is still down:

BUG: unable to handle kernel NULL pointer dereference at 0000000000000010
IP: [<ffffffffa0170fb2>] ixgbe_alloc_rx_buffers+0x42/0x1a0 [ixgbe]

Call Trace:
[<ffffffffa017336b>] ixgbe_configure_rx_ring+0x2eb/0x3d0 [ixgbe]
[<ffffffffa0173811>] ixgbe_fwd_ring_up+0xd1/0x380 [ixgbe]
[<ffffffffa0179709>] ixgbe_fwd_add+0x149/0x230 [ixgbe]
[<ffffffffa0113480>] macvlan_open+0x260/0x2b0 [macvlan]

Reported-by: Matthew Garrett <mjg59@coreos.com>
Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>

drivers/net/ethernet/intel/ixgbe/ixgbe_main.c

@@ -9135,10 +9135,14 @@ static void *ixgbe_fwd_add(struct net_device *pdev, struct net_device *vdev)
 		goto fwd_add_err;
 	fwd_adapter->pool = pool;
 	fwd_adapter->real_adapter = adapter;
-	err = ixgbe_fwd_ring_up(vdev, fwd_adapter);
-	if (err)
-		goto fwd_add_err;
-	netif_tx_start_all_queues(vdev);
+
+	if (netif_running(pdev)) {
+		err = ixgbe_fwd_ring_up(vdev, fwd_adapter);
+		if (err)
+			goto fwd_add_err;
+		netif_tx_start_all_queues(vdev);
+	}
+
 	return fwd_adapter;
 fwd_add_err:
 	/* unwind counter and free adapter struct */