vhost-net: unbreak busy polling

Commit 67a873df ("vhost: basic in order support") pass the number
of used elem to vhost_net_rx_peek_head_len() to make sure it can
signal the used correctly before trying to do busy polling. But it
forgets to clear the count, this would cause the count run out of sync
with handle_rx() and break the busy polling.

Fixing this by passing the pointer of the count and clearing it after
the signaling the used.

Acked-by: Michael S. Tsirkin <mst@redhat.com>
Cc: stable@vger.kernel.org
Fixes: 67a873df ("vhost: basic in order support")
Signed-off-by: Jason Wang <jasowang@redhat.com>
Message-Id: <20250917063045.2042-1-jasowang@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>