vhost_net: Rename local variables in vhost_net_rx_peek_head_len

So we can easily see which variable is for which, tx or rx.

Signed-off-by: Toshiaki Makita <makita.toshiaki@lab.ntt.co.jp>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>