vhost: Added pad cleanup if vnet_hdr is not present.

When the Qemu launched with vhost but without tap vnet_hdr,
vhost tries to copy vnet_hdr from socket iter with size 0
to the page that may contain some trash.
That trash can be interpreted as unpredictable values for
vnet_hdr.
That leads to dropping some packets and in some cases to
stalling vhost routine when the vhost_net tries to process
packets and fails in a loop.

Qemu options:
  -netdev tap,vhost=on,vnet_hdr=off,...
Signed-off-by: Andrew Melnychenko <andrew@daynix.com>
Message-Id: <20240115194840.1183077-1-andrew@daynix.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/vhost/net.c

@@ -735,6 +735,9 @@ static int vhost_net_build_xdp(struct vhost_net_virtqueue *nvq,
 	hdr = buf;
 	gso = &hdr->gso;
 
+	if (!sock_hlen)
+		memset(buf, 0, pad);
+
 	if ((gso->flags & VIRTIO_NET_HDR_F_NEEDS_CSUM) &&
 	    vhost16_to_cpu(vq, gso->csum_start) +
 	    vhost16_to_cpu(vq, gso->csum_offset) + 2 >