CVE-2026-74440: drm/xe: Wait on external BO kernel fences in exec IOCTL

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

drm/xe: Wait on external BO kernel fences in exec IOCTL

Before arming a user job, xeexecioctl() only added the VM's dma-resv KERNEL slot as a dependency. That slot covers rebinds and the kernel operations of the VM's private BOs, but not external BOs (bo->vm == NULL), which carry their kernel operations (evictions, moves, ...) in their own dma-resv KERNEL slot.

The DMARESVUSAGEKERNEL slot is the cross-driver contract for memory management operations that must complete before the BO or its backing store may be used: any accessor is required to wait on the KERNEL fences before touching the resv. By skipping the external BOs' KERNEL slots, the exec path violated that contract and could schedule a user job while a kernel operation on an external BO mapped by the VM was still in flight, racing against it and potentially reading or writing memory that was being moved.

Replace the VM-only dependency with an iteration over every object locked by the exec, adding each object's KERNEL slot as a job dependency. This covers the VM resv (rebinds and private BOs) as well as every external BO, mirroring the drmgpuvmresvaddfence() call that later publishes the job fence to the same set of objects. Long-running mode continues to skip this, as before.

(cherry picked from commit a6b842acf3ddd1efc53a56de9260cfa718fb35e7)