vhost: remove unnecessary memset when enqueueing

We have to reset the virtio net hdr at virtio_enqueue_offload()
before, due to all mbufs share a single virtio_hdr structure:

	struct virtio_net_hdr_mrg_rxbuf virtio_hdr = {{0, }, 0};

	foreach (mbuf) {
		virtio_enqueue_offload(mbuf, &virtio_hdr.hdr);

		copy net hdr and mbuf to desc buf
	}

However, after the vhost rxtx refactor, the code looks like:

	copy_mbuf_to_desc(mbuf)
	{
		struct virtio_net_hdr_mrg_rxbuf virtio_hdr = {{0, }, 0}

		virtio_enqueue_offload(mbuf, &virtio_hdr.hdr);

		copy net hdr and mbuf to desc buf
	}

	foreach (mbuf) {
		copy_mbuf_to_desc(mbuf);
	}

Therefore, the memset at virtio_enqueue_offload() is not necessary
any more; remove it.

Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Acked-by: Huawei Xie <huawei.xie@intel.com>