examples/vhost: fix performance

examples/vhost: fix performance

We find significant perfermance drop introduced by below commit,
when vhost example is started with --mergeable 0 and inside vm,
kernel virtio-net driver is used to do ip based forwarding.

The commit, 859b480d5afd ("vhost: add guest offload setting"), adds
support for VIRTIO_NET_F_GUEST_TSO4 and VIRTIO_NET_F_GUEST_TSO6,
in vhost lib. But inside vhost example, the way to disable tso only
excludes the direction from virtio to vhost, but not the opposite
direction. When mergeable is disabled, it triggers big_packets path
of virtio-net driver to prepare to receive possible big packets with
size of 64K. Because mergeable is off, for each entry of avail ring,
virtio driver uses 19 desc chained together, with one desc pointing
to header, other 18 desc pointing to 4K-sized pages. But QEMU only
creates 256 desc entries for each vq, which results in that only 13
packets can be received. VM kernel can quickly handle those packets
and go to sleep (HLT).

As QEMU has no option to set the desc entries of a vq, so here,
we disable VIRTIO_NET_F_GUEST_TSO4 and VIRTIO_NET_F_GUEST_TSO6
with VIRTIO_NET_F_HOST_TSO4 and VIRTIO_NET_F_HOST_TSO6 when we
disable tso of vhost example, to avoid VM kernel virtio driver
go into big_packets path.

Fixes: 9fd72e3cbd29 ("examples/vhost: add virtio offload")

Reported-by: Qian Xu <qian.q.xu@intel.com>
Signed-off-by: Jianfeng Tan <jianfeng.tan@intel.com>
Tested-by: Qian Xu <qian.q.xu@intel.com>
Acked-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>