vsock/virtio: fix `rx_bytes` accounting for stream sockets

vsock/virtio: fix `rx_bytes` accounting for stream sockets

In `struct virtio_vsock_sock`, we maintain two counters:
- `rx_bytes`: used internally to track how many bytes have been read.
  This supports mechanisms like .stream_has_data() and sock_rcvlowat().
- `fwd_cnt`: used for the credit mechanism to inform available receive
  buffer space to the remote peer.

These counters are updated via virtio_transport_inc_rx_pkt() and
virtio_transport_dec_rx_pkt().

Since the beginning with commit 06a8fc78367d ("VSOCK: Introduce
virtio_vsock_common.ko"), we call virtio_transport_dec_rx_pkt() in
virtio_transport_stream_do_dequeue() only when we consume the entire
packet, so partial reads, do not update `rx_bytes` and `fwd_cnt`.

This is fine for `fwd_cnt`, because we still have space used for the
entire packet, and we don't want to update the credit for the other
peer until we free the space of the entire packet. However, this
causes `rx_bytes` to be stale on partial reads.

Previously, this didn’t cause issues because `rx_bytes` was used only by
.stream_has_data(), and any unread portion of a packet implied data was
still available. However, since commit 93b808876682
("virtio/vsock: fix logic which reduces credit update messages"), we now
rely on `rx_bytes` to determine if a credit update should be sent when
the data in the RX queue drops below SO_RCVLOWAT value.

This patch fixes the accounting by updating `rx_bytes` with the number
of bytes actually read, even on partial reads, while leaving `fwd_cnt`
untouched until the packet is fully consumed. Also introduce a new
`buf_used` counter to check that the remote peer is honoring the given
credit; this was previously done via `rx_bytes`.

Fixes: 93b808876682 ("virtio/vsock: fix logic which reduces credit update messages")
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://patch.msgid.link/20250521121705.196379-1-sgarzare@redhat.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>