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

Before arming a user job, xe_exec_ioctl() 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 DMA_RESV_USAGE_KERNEL 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 drm_gpuvm_resv_add_fence() call
that later publishes the job fence to the same set of objects.
Long-running mode continues to skip this, as before.

Fixes: dd08ebf6c352 ("drm/xe: Introduce a new DRM driver for Intel GPUs")
Cc: stable@vger.kernel.org
Assisted-by: GitHub_Copilot:claude-opus-4.8
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patch.msgid.link/20260702215805.4011228-1-matthew.brost@intel.com

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