dma-fence: Silence sparse warning in dma_fence_describe

Sparse complains about assigning a string to a __rcu annotated local
variable:

drivers/dma-buf/dma-fence.c:1040:38: warning: incorrect type in initializer (different address spaces)
drivers/dma-buf/dma-fence.c:1040:38:    expected char const [noderef] __rcu *timeline
drivers/dma-buf/dma-fence.c:1040:38:    got char *
drivers/dma-buf/dma-fence.c:1041:36: warning: incorrect type in initializer (different address spaces)
drivers/dma-buf/dma-fence.c:1041:36:    expected char const [noderef] __rcu *driver
drivers/dma-buf/dma-fence.c:1041:36:    got char *

It is harmless but lets silence it.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Fixes: ac36401 ("dma-buf: cleanup dma_fence_describe v3")
Cc: Christian König <christian.koenig@amd.com>
Cc: linux-media@vger.kernel.org
Cc: dri-devel@lists.freedesktop.org
Cc: linaro-mm-sig@lists.linaro.org
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Tvrtko Ursulin <tursulin@ursulin.net>
Link: https://lore.kernel.org/r/20260415083207.40513-1-tvrtko.ursulin@igalia.com

Author: Tvrtko Ursulin

drivers/dma-buf/dma-fence.c

@@ -1021,8 +1021,8 @@ EXPORT_SYMBOL(dma_fence_set_deadline);
  */
 void dma_fence_describe(struct dma_fence *fence, struct seq_file *seq)
 {
-	const char __rcu *timeline = "";
-	const char __rcu *driver = "";
+	const char __rcu *timeline = (const char __rcu *)"";
+	const char __rcu *driver = (const char __rcu *)"";
 	const char *signaled = "";
 
 	rcu_read_lock();