net: kcm: Fix race condition in kcm_unattach()

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

net: kcm: Fix race condition in kcm_unattach()

syzbot found a race condition when kcm_unattach(psock) and kcm_release(kcm) are executed at the same time.

kcm_unattach() is missing a check of the flag kcm->tx_stopped before calling queue_work().

If the kcm has a reserved psock, kcm_unattach() might get executed between cancel_work_sync() and unreserve_psock() in kcm_release(), requeuing kcm->tx_work right before kcm gets freed in kcm_done().

Remove kcm->tx_stopped and replace it by the less error-prone disable_work_sync().