spi: Fix memory leak on splited transfers

In the prepare_message callback the bus driver has the
opportunity to split a transfer into smaller chunks.
spi_map_msg is done after prepare_message.

Function spi_res_release releases the splited transfers
in the message. Therefore spi_res_release should be called
after spi_map_msg.

The previous try at this was commit c9ba7a16
which released the splited transfers after
spi_finalize_current_message had been called.
This introduced a race since the message struct could be
out of scope because the spi_sync call got completed.

Fixes this leak on spi bus driver spi-bcm2835.c when transfer
size is greater than 65532:

Kmemleak:
sg_alloc_table+0x28/0xc8
spi_map_buf+0xa4/0x300
__spi_pump_messages+0x370/0x748
__spi_sync+0x1d4/0x270
spi_sync+0x34/0x58
spi_test_execute_msg+0x60/0x340 [spi_loopback_test]
spi_test_run_iter+0x548/0x578 [spi_loopback_test]
spi_test_run_test+0x94/0x140 [spi_loopback_test]
spi_test_run_tests+0x150/0x180 [spi_loopback_test]
spi_loopback_test_probe+0x50/0xd0 [spi_loopback_test]
spi_drv_probe+0x84/0xe0
Signed-off-by: Gustav Wiklander <gustavwi@axis.com>
Link: https://lore.kernel.org/r/20200908151129.15915-1-gustav.wiklander@axis.comSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi.c

@@ -1327,8 +1327,6 @@ static int spi_transfer_one_message(struct spi_controller *ctlr,
 	if (msg->status && ctlr->handle_err)
 		ctlr->handle_err(ctlr, msg);
 
-	spi_res_release(ctlr, msg);
-
 	spi_finalize_current_message(ctlr);
 
 	return ret;
@@ -1727,6 +1725,13 @@ void spi_finalize_current_message(struct spi_controller *ctlr)
 
 	spi_unmap_msg(ctlr, mesg);
 
+	/* In the prepare_messages callback the spi bus has the opportunity to
+	 * split a transfer to smaller chunks.
+	 * Release splited transfers here since spi_map_msg is done on the
+	 * splited transfers.
+	 */
+	spi_res_release(ctlr, mesg);
+
 	if (ctlr->cur_msg_prepared && ctlr->unprepare_message) {
 		ret = ctlr->unprepare_message(ctlr, mesg);
 		if (ret) {