dma-buf: handle testing kthreads creation failure

commit 6cb05d89 upstream.

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.ru
Signed-off-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>