vhost-user: fix VIRTIO_NET_F_MRG_RXBUF negotiation

Header length check should happen only if backend is kernel. For user
backend there is no reason to reset this bit.

vhost-user code does not define .has_vnet_hdr_len so
VIRTIO_NET_F_MRG_RXBUF cannot be negotiated even if both sides
support it.

Signed-off-by: Damjan Marion <damarion@cisco.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
(cherry picked from commit d8e80ae)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

hw/net/vhost_net.c

@@ -163,11 +163,11 @@ struct vhost_net *vhost_net_init(VhostNetOptions *options)
     if (r < 0) {
         goto fail;
     }
-    if (!qemu_has_vnet_hdr_len(options->net_backend,
-                               sizeof(struct virtio_net_hdr_mrg_rxbuf))) {
-        net->dev.features &= ~(1 << VIRTIO_NET_F_MRG_RXBUF);
-    }
     if (backend_kernel) {
+        if (!qemu_has_vnet_hdr_len(options->net_backend,
+                               sizeof(struct virtio_net_hdr_mrg_rxbuf))) {
+            net->dev.features &= ~(1 << VIRTIO_NET_F_MRG_RXBUF);
+        }
         if (~net->dev.features & net->dev.backend_features) {
             fprintf(stderr, "vhost lacks feature mask %" PRIu64
                    " for backend\n",