drm/sched: Fix fence reference count leak

The last_scheduled fence leaks when an entity is being killed and adding
the cleanup callback fails.

Decrement the reference count of prev when dma_fence_add_callback()
fails, ensuring proper balance.

Cc: stable@vger.kernel.org	# v6.2+
[phasta: add git tag info for stable kernel]
Fixes: 2fdb8a8 ("drm/scheduler: rework entity flush, kill and fini")
Signed-off-by: qianyi liu <liuqianyi125@gmail.com>
Signed-off-by: Philipp Stanner <phasta@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20250311060251.4041101-1-liuqianyi125@gmail.com

drivers/gpu/drm/scheduler/sched_entity.c

@@ -259,9 +259,16 @@ static void drm_sched_entity_kill(struct drm_sched_entity *entity)
 		struct drm_sched_fence *s_fence = job->s_fence;
 
 		dma_fence_get(&s_fence->finished);
-		if (!prev || dma_fence_add_callback(prev, &job->finish_cb,
-					   drm_sched_entity_kill_jobs_cb))
+		if (!prev ||
+		    dma_fence_add_callback(prev, &job->finish_cb,
+					   drm_sched_entity_kill_jobs_cb)) {
+			/*
+			 * Adding callback above failed.
+			 * dma_fence_put() checks for NULL.
+			 */
+			dma_fence_put(prev);
 			drm_sched_entity_kill_jobs_cb(NULL, &job->finish_cb);
+		}
 
 		prev = &s_fence->finished;
 	}