CVE-2026-52951: drm/xe/dma-buf: handle empty bo and UAF races

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

drm/xe/dma-buf: handle empty bo and UAF races

There look to be some nasty races here when triggering the invalidatemappings hook:

1) We do xeboalloc() followed by the attach, before the actual full bo init step in xedmabufinitobj(). However the bo is visible on the attachments list after the attach. This is bad since exporter driver, say amdgpu, can at any time call back into our invalidatemappings hook, with an empty/bogus bo, leading to potential bugs/crashes.

2) Similar to 1) but here we get a UAF, when the invalidatemappings hook is triggered. For example, we get as far as xeboinitlocked() but this fails in some way. But here the bo will be freed on error, but we still have it attached from dma-buf pov, so if the invalidatemappings is now triggered then the bo we access is gone and we trigger UAF and more bugs/crashes.

To fix this, move the attach step until after we actually have a fully set up buffer object. Note that the bo is not published to userspace until later, so not sure what the comment "Don't publish the bo until we have a valid attachment", is referring to.

We have at least two different customers reporting hitting a NULL ptr deref in evictflags when importing something from amdgpu, followed by triggering the evict flow. Hit rate is also pretty low, which would hint at some kind of race, so something like 1) or 2) might explain this.

v2: - Shuffle the order of the ops slightly (no functional change) - Improve the comment to better explain the ordering (Matt B)

(cherry picked from commit af1f2ad0c59fe4e2f924c526f66e968289d77971)