drm/xe/vm: nuke PTs only after unlinking contested VMAs

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

drm/xe/vm: nuke PTs only after unlinking contested VMAs

In xe_vm_close_and_put(), external-BO VMAs are queued on the contested
list for deferred destruction via xe_vma_destroy_unlocked(). However,
xe_vm_pt_destroy() was previously invoked before processing contested
VMAs, destroying vm->pt_root while those VMAs were still linked to their
respective buffer objects (vm_bo->list.gpuva).

If a concurrent thread evicts one of those shared buffer objects,
xe_bo_trigger_rebind() holding only bo->resv walks the BO's VMAs and, in
fault mode, calls xe_vm_invalidate_vma() -> xe_pt_zap_ptes(). Because
vm->pt_root[tile->id] is already NULL, dereferencing pt->level causes a
NULL ptr deref.

Fix this by deferring xe_vm_free_scratch() and xe_vm_pt_destroy() until
after all contested VMAs have been unlinked and destroyed.

User is reporting hitting a NULL ptr deref in xe_pt_zap_ptes(), which
could be explained by this race.

(cherry picked from commit c2863648959489767f08892fd6e90577d2ea0b6a)