vhost_net: stop device during reset owner

We don't stop device before reset owner, this means we could try to
serve any virtqueue kick before reset dev->worker. This will result a
warn since the work was pending at llist during owner resetting. Fix
this by stopping device during owner reset.

Reported-by:  <syzbot+eb17c6162478cc50632c@syzkaller.appspotmail.com>
Fixes: 3a4d5c94

 ("vhost_net: a kernel-level virtio server")
Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>