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 - in drivers/vhost/net.c and when mergeable buffers have been enabled, the driver checks whether the combined entries are big enough to store an incoming packet. If not, the driver re-enables notifications with available entries still in the ring. The incorrect return value from vhostgetavailidx propagates through vhostenablenotify and causes the host to livelock if the guest is not making progress, as vhost will immediately disable notifications and retry using the available entries.

This goes back to commit d3bb267bbdcb ("vhost: cache avail index in vhostenablenotify()") which changed vhostenablenotify() to compare the freshly read avail index against vq->lastavailidx instead of the previously cached vq->availidx. Commit 7ad472397667 ("vhost: move smprmb() into vhostgetavailidx()") then carried over the same comparison when refactoring vhostenablenotify() to call the unified vhostgetavailidx().

The obvious fix is to make vhostgetavailidx do what the comment says it does and report whether new entries have been added.