vhost_net: introduce vhost_exceeds_weight()

commit 272f35cba53d088085e5952fd81d7a133ab90789 upstream.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Balbir Singh <sblbir@amzn.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>