spi: fix use-after-free of the add_lock mutex

commit 6c53b45c71 upstream.

Commit 6098475d4c ("spi: Fix deadlock when adding SPI controllers on
SPI buses") introduced a per-controller mutex. But mutex_unlock() of
said lock is called after the controller is already freed:

  spi_unregister_controller(ctlr)
  -> put_device(&ctlr->dev)
    -> spi_controller_release(dev)
  -> mutex_unlock(&ctrl->add_lock)

Move the put_device() after the mutex_unlock().

Fixes: 6098475d4c ("spi: Fix deadlock when adding SPI controllers on SPI buses")
Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Lukas Wunner <lukas@wunner.de>
Cc: stable@vger.kernel.org # v5.15
Link: https://lore.kernel.org/r/20211111083713.3335171-1-michael@walle.cc
Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/spi/spi.c

@@ -3020,12 +3020,6 @@ void spi_unregister_controller(struct spi_controller *ctlr)
 
 	device_del(&ctlr->dev);
 
-	/* Release the last reference on the controller if its driver
-	 * has not yet been converted to devm_spi_alloc_master/slave().
-	 */
-	if (!ctlr->devm_allocated)
-		put_device(&ctlr->dev);
-
 	/* free bus id */
 	mutex_lock(&board_lock);
 	if (found == ctlr)
@@ -3034,6 +3028,12 @@ void spi_unregister_controller(struct spi_controller *ctlr)
 
 	if (IS_ENABLED(CONFIG_SPI_DYNAMIC))
 		mutex_unlock(&ctlr->add_lock);
+
+	/* Release the last reference on the controller if its driver
+	 * has not yet been converted to devm_spi_alloc_master/slave().
+	 */
+	if (!ctlr->devm_allocated)
+		put_device(&ctlr->dev);
 }
 EXPORT_SYMBOL_GPL(spi_unregister_controller);