vsock: Update rx_bytes on read_skb()

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

vsock: Update rx_bytes on read_skb()

Make sure virtio_transport_inc_rx_pkt() and virtio_transport_dec_rx_pkt()
calls are balanced (i.e. virtio_vsock_sock::rx_bytes doesn't lie) after
vsock_transport::read_skb().

While here, also inform the peer that we've freed up space and it has more
credit.

Failing to update rx_bytes after packet is dequeued leads to a warning on
SOCK_STREAM recv():

[ 233.396654] rx_queue is empty, but rx_bytes is non-zero
[ 233.396702] WARNING: CPU: 11 PID: 40601 at net/vmw_vsock/virtio_transport_common.c:589