CVE-2026-68384: drm/xe/vf: Fix VF CCS attach/detach race with in-flight BO moves

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

drm/xe/vf: Fix VF CCS attach/detach race with in-flight BO moves

xebomove() attaches VF CCS read/write batch buffers (BBs) to a BO after it transitions NULL/SYSTEM -> TT, and detaches them after it transitions TT -> SYSTEM. Both operations were done synchronously on the CPU immediately after building the move's copy/clear fence, without waiting for that fence to signal. This creates two races with VF migration:

- Attach happens too late relative to the copy job it is meant to protect. If the copy job is submitted before the CCS BBs are attached, a VF migration event that pauses execution mid-copy can observe partially copied CCS metadata without the attach state needed to correctly save/restore it.

- Detach happens too early relative to the copy job that moves data out of TT. The CCS BBs are torn down right after the copy fence is obtained, while the actual blit may still be in flight. A VF migration event that pauses execution mid-copy can then race the save/restore path against the still-running blit, and the CCS BBs it would need to make sense of the paused state have already been removed.

Fix both races:

- Move the attach call to before the copy/clear job is submitted, so the CCS BBs are already registered by the time the copy runs. On attach failure, unwind and bail out of the move. xemigrateccsrwcopy() now takes the destination resource explicitly, since bo->ttm.resource is not updated to the new resource until after the move commits.

- Detach only after explicitly waiting for the copy fence to signal, instead of tearing down the CCS BBs immediately after obtaining it.

While here, also fix xesriovvfccsattachbo() to properly unwind and propagate errors: the per-context loop previously never broke out on error, silently discarding earlier failures. Unwind by clearing each attached context directly via xemigrateccsrwcopyclear() instead of reusing xesriovvfccsdetachbo(), which requires both contexts to be attached before it will clean up either one.

(cherry picked from commit d45ad0aa7a1eb5d7288b5ed948b05695611dc39e)