CVE-2026-74328: iommufd: Destroy the pages content after detaching from dmabuf

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

iommufd: Destroy the pages content after detaching from dmabuf

Sashiko points out this has gotten out of order, the mutex could still be in use through the dmabuf invalidation callbacks. Don't destroy any of the pages content until the dmabuf is fully detached.