vsock/virtio: free queued packets when closing socket

As reported by syzbot [1], there is a memory leak while closing the
socket. We partially solved this issue with commit ac03046e
("vsock/virtio: free packets during the socket release"), but we
forgot to drain the RX queue when the socket is definitely closed by
the scheduled work.

To avoid future issues, let's use the new virtio_transport_remove_sock()
to drain the RX queue before removing the socket from the af_vsock lists
calling vsock_remove_sock().

[1] https://syzkaller.appspot.com/bug?extid=24452624fc4c571eedd9

Fixes: ac03046e

 ("vsock/virtio: free packets during the socket release")
Reported-and-tested-by:  <syzbot+24452624fc4c571eedd9@syzkaller.appspotmail.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>