vsock/virtio: account only unread bytes in read_skb()

In the Linux kernel, the following vulnerability has been resolved: vsock/virtio: account only unread bytes in read_skb() After a partial stream receive, rx_bytes tracks the unread suffix while buf_used and peer credit still cover the whole packet. virtio_transport_read_skb() dequeues that packet but passes pkt_len for both counters, underflowing rx_bytes. On a connected CID_LOCAL stream with an SK_SKB verdict, a 40-byte read from a 100-byte packet made SIOCINQ report -40, an empty recv return ELOOP, and poll report the empty socket readable. After verdict detach, receiving 40 bytes wrapped the counter to zero and hid those queued bytes until one more byte arrived. The fixed run kept the counter balanced and exposed all queued data immediately. This was found during an LLM-assisted manual source audit of VSOCK receive accounting while re-evaluating virtio_transport_read_skb() after CVE-2024-50169. Subtract only skb->len minus the VSOCK offset from rx_bytes. Retain the full packet length for buf_used and peer credit.