vhost: introduce vhost_exceeds_weight()

mainline inclusion
from mainline-5.2-rc2
commit e82b9b07
category: bugfix
bugzilla: 13690
CVE: CVE-2019-3900

-------------------------------------------------

We used to have vhost_exceeds_weight() for vhost-net to:

- prevent vhost kthread from hogging the cpu
- balance the time spent between TX and RX

This function could be useful for vsock and scsi as well. So move it
to vhost.c. Device must specify a weight which counts the number of
requests, or it can also specific a byte_weight which counts the
number of bytes that has been processed.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Conflicts:
  drivers/vhost/net.c
[yyl: adjust context]

Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Zhen Lei <thunder.leizhen@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>