vsock/virtio: fix potential underflow in virtio_transport_get_credit()

In the Linux kernel, the following vulnerability has been resolved:

vsock/virtio: fix potential underflow in virtio_transport_get_credit()

The credit calculation in virtio_transport_get_credit() uses unsigned arithmetic:

ret = vvs->peer_buf_alloc - (vvs->tx_cnt - vvs->peer_fwd_cnt);

If the peer shrinks its advertised buffer (peer_buf_alloc) while bytes are in flight, the subtraction can underflow and produce a large positive value, potentially allowing more data to be queued than the peer can handle.

Reuse virtio_transport_has_space() which already handles this case and add a comment to make it clear why we are doing that.

[Stefano: use virtio_transport_has_space() instead of duplicating the code] [Stefano: tweak the commit message]