Medium [CVE-2026-74312] validate virtqueue index in mmap and fault paths

In the Linux kernel, the following vulnerability has been resolved: vhost/vdpa: validate virtqueue index in mmap and fault paths vhost_vdpa_mmap() and vhost_vdpa_fault() use vma->vm_pgoff as a virtqueue index for get_vq_notification(), but they do not validate that the index is smaller than v->nvqs.

The ioctl path already performs both a bounds check and array_index_nospec(), but the mmap/fault path only checks that the index fits in u16. This allows an out-of-range queue index to reach driver-specific get_vq_notification() callbacks.

Fix this by extracting a unified vhost_vdpa_get_vq_notification() helper that validates the queue index against v->nvqs and applies array_index_nospec() before calling the driver callback. Both the mmap and fault paths use this helper, and the bounds checking is consolidated into a single location.

From source inspection, the most defensible impact is out-of-bounds access in the callback path, potentially leading to invalid PFN remaps and crash/DoS. Improper validation of the virtqueue index in the `vhost_vdpa_mmap()` and `vhost_vdpa_fault()` functions allows an out-of-range index to be used.

This can lead to an out-of-bounds memory access in driver-specific callbacks, potentially causing invalid Page Frame Number (PFN) remaps, system crashes, or a Denial of Service (DoS).