virtio-net: Add validation for used length

This adds validation for used length (might come
from an untrusted device) to avoid data corruption
or loss.

Signed-off-by: Xie Yongji <xieyongji@bytedance.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20210531135852.113-1-xieyongji@bytedance.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>