vhost_net: get rid of vhost_net_flush_vq() and extra flush calls

vhost_net_flush_vq() calls vhost_work_dev_flush() twice passing
vhost_dev pointer obtained via 'n->poll[index].dev' and
'n->vqs[index].vq.poll.dev'. This is actually the same pointer,
initialized in vhost_net_open()/vhost_dev_init()/vhost_poll_init()

Remove vhost_net_flush_vq() and call vhost_work_dev_flush() directly.
Do the flushes only once instead of several flush calls in a row
which seems rather useless.

Signed-off-by: Andrey Ryabinin <arbn@yandex-team.com>
[drop vhost_dev forward declaration in vhost.h]
Signed-off-by: Mike Christie <michael.christie@oracle.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Message-Id: <20220517180850.198915-4-michael.christie@oracle.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>