dma-buf: handle testing kthreads creation failure

kthread creation may possibly fail inside race_signal_callback(). In
such a case stop the already started threads, put the already taken
references to them and return with error code.

Found by Linux Verification Center (linuxtesting.org).

Fixes: 2989f645 ("dma-buf: Add selftests for dma-fence")
Cc: stable@vger.kernel.org
Signed-off-by: Fedor Pchelkin <pchelkin@ispras.ru>
Reviewed-by: T.J. Mercier <tjmercier@google.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240522181308.841686-1-pchelkin@ispras.ruSigned-off-by: Christian König <christian.koenig@amd.com>

drivers/dma-buf/st-dma-fence.c

@@ -540,6 +540,12 @@ static int race_signal_callback(void *arg)
 			t[i].before = pass;
 			t[i].task = kthread_run(thread_signal_callback, &t[i],
 						"dma-fence:%d", i);
+			if (IS_ERR(t[i].task)) {
+				ret = PTR_ERR(t[i].task);
+				while (--i >= 0)
+					kthread_stop_put(t[i].task);
+				return ret;
+			}
 			get_task_struct(t[i].task);
 		}