CVE-2026-98518: drm/nouveau/dmem: pin VRAM for the whole registered range

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

drm/nouveau/dmem: pin VRAM for the whole registered range

Commit c32287471077 ("gpu/drm/nouveau: enable THP support for GPU memory migration") grew the device-private region that nouveaudmemchunkalloc() registers from DMEMCHUNKSIZE to DMEMCHUNKSIZE NRCHUNKS, but left the VRAM buffer object backing that region at DMEMCHUNKSIZE.

nouveaudmempageaddr() returns chunk->bo->offset plus the page's offset within the registered region, so every page past the first chunk resolves to VRAM outside the buffer object.

Size the buffer object to the region it backs.