vhost: add dequeue zero copy

vhost: add dequeue zero copy

The basic idea of dequeue zero copy is, instead of copying data from
the desc buf, here we let the mbuf reference the desc buf addr directly.

Doing so, however, has one major issue: we can't update the used ring
at the end of rte_vhost_dequeue_burst. Because we don't do the copy
here, an update of the used ring would let the driver to reclaim the
desc buf. As a result, DPDK might reference a stale memory region.

To update the used ring properly, this patch does several tricks:

- when mbuf references a desc buf, refcnt is added by 1.

  This is to pin lock the mbuf, so that a mbuf free from the DPDK
  won't actually free it, instead, refcnt is subtracted by 1.

- We chain all those mbuf together (by tailq)

  And we check it every time on the rte_vhost_dequeue_burst entrance,
  to see if the mbuf is freed (when refcnt equals to 1). If that
  happens, it means we are the last user of this mbuf and we are
  safe to update the used ring.

- "struct zcopy_mbuf" is introduced, to associate an mbuf with the
  right desc idx.

Dequeue zero copy is introduced for performance reason, and some rough
tests show about 50% perfomance boost for packet size 1500B. For small
packets, (e.g. 64B), it actually slows a bit down (well, it could up to
15%). That is expected because this patch introduces some extra works,
and it outweighs the benefit from saving few bytes copy.

Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Tested-by: Qian Xu <qian.q.xu@intel.com>