CVE-2026-74356 vhost: fix vhost_get_avail_idx for a non empty ring

In the Linux kernel, the following vulnerability has been resolved: vhost: fix vhostgetavailidx for a non empty ring vhostgetavailidx is supposed to report whether it has updated vq-availidx. Instead, it returns whether all entries have been consumed, which is usually the same. But not always - i...