xdp: check device pointer before clearing

We should not call 'ndo_bpf()' or 'dev_put()' with NULL argument.

Fixes: c9b47cc1 ("xsk: fix bug when trying to use both copy and zero-copy on one queue id")
Signed-off-by: Ilya Maximets <i.maximets@samsung.com>
Acked-by: Jonathan Lemon <jonathan.lemon@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

net/xdp/xdp_umem.c

@@ -143,6 +143,9 @@ static void xdp_umem_clear_dev(struct xdp_umem *umem)
 	struct netdev_bpf bpf;
 	int err;
 
+	if (!umem->dev)
+		return;
+
 	if (umem->zc) {
 		bpf.command = XDP_SETUP_XSK_UMEM;
 		bpf.xsk.umem = NULL;
@@ -156,11 +159,9 @@ static void xdp_umem_clear_dev(struct xdp_umem *umem)
 			WARN(1, "failed to disable umem!\n");
 	}
 
-	if (umem->dev) {
 	rtnl_lock();
 	xdp_clear_umem_at_qid(umem->dev, umem->queue_id);
 	rtnl_unlock();
-	}
 
 	if (umem->zc) {
 		dev_put(umem->dev);