drm/xe: Use local fence in error path of xe_migrate_clear

The intent of the error path in xe_migrate_clear is to wait on locally
generated fence and then return. The code is waiting on m->fence which
could be the local fence but this is only stable under the job mutex
leading to a possible UAF. Fix code to wait on local fence.

Fixes: dd08ebf ("drm/xe: Introduce a new DRM driver for Intel GPUs")
Cc: stable@vger.kernel.org
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://lore.kernel.org/r/20250311182915.3606291-1-matthew.brost@intel.com

drivers/gpu/drm/xe/xe_migrate.c

@@ -1177,7 +1177,7 @@ struct dma_fence *xe_migrate_clear(struct xe_migrate *m,
 err_sync:
 		/* Sync partial copies if any. FIXME: job_mutex? */
 		if (fence) {
-			dma_fence_wait(m->fence, false);
+			dma_fence_wait(fence, false);
 			dma_fence_put(fence);
 		}