can: isotp: fix timer drain order, wakeup handling and tx_gen ordering

In the Linux kernel, the following vulnerability has been resolved:

can: isotp: fix timer drain order, wakeup handling and tx_gen ordering

This patch is a follow-up to commit cf070fe33bfb ("can: isotp: serialize
TX state transitions under so->rx_lock") which addresses following
sashiko-bot findings:

- isotp_sendmsg(): drain so->txfrtimer first so a stale callback can't
re-arm echotimer after the claim

- isotp_release(): wake so->wait after forcing ISOTP_SHUTDOWN so a
sleeping sendmsg() claim isn't stranded

- isotp_sendmsg(): have both wait_event_interruptible() calls in
isotp_sendmsg() also wake on ISOTP_SHUTDOWN and do not return claim to
IDLE to avoid corrupting a concurrent isotp_release() process.

- isotp_sendmsg(): handle potential claim of a new transfer when
the wait_event_interruptible() call returns in CAN_ISOTP_WAIT_TX_DONE
mode. Don't touch timers and states of the new transfer if a new thread
incremented so->tx_gen before getting the lock at err_event_drop.

- isotp_sendmsg(): handle a stuck can_send() and omit timer and state
changes if a new transfer was claimed. wait_tx_done() returns the error
recorded in so->tx_result[], tagged with the caller's own generation.

- isotp_tx_timeout(): on a claimed timeout, record the ECOMM error for
the timed-out transfer's own generation in so->tx_result[]; sk->sk_err
is raised unconditionally, same as every other error path here.

- isotp_tx_gen_done()/isotp_tx_timeout(): always read tx.state (acquire)
before tx_gen - the reverse order let a weakly ordered CPU pair a fresh
tx.state with a stale tx_gen/tx_result slot.

- isotp_sendmsg(): wait_tx_done: drain sk_err via sock_error() once we
have read the result from so->tx_result[], so an already-reported error
doesn't stay latched for a later poll()/SO_ERROR.

Also align the remaining lock-free so->tx.state/rx.state/cfecho accesses
and use skb->hash as unique loopback echo frame indicator.