When isotp_sendmsg() concurrent, tx.state of all TX processes can be
ISOTP_IDLE. The conditions so->tx.state != ISOTP_IDLE and
wq_has_sleeper(&so->wait) can not protect TX buffer from being
accessed by multiple TX processes.

We can use cmpxchg() to try to modify tx.state to ISOTP_SENDING firstly.
If the modification of the previous process succeed, the later process
must wait tx.state to ISOTP_IDLE firstly. Thus, we can ensure TX buffer
is accessed by only one process at the same time. And we should also
restore the original tx.state at the subsequent error processes.

Fixes: e057dd3f ("can: add ISO 15765-2:2016 transport protocol")
Link: https://lore.kernel.org/all/c2517874fbdf4188585cf9ddf67a8fa74d5dbde5.1633764159.git.william.xuanziyang@huawei.com
Cc: stable@vger.kernel.org
Signed-off-by: Ziyang Xuan <william.xuanziyang@huawei.com>
Acked-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>