CVE-2026-98754: tty: vcc: hold port lock when clearing tty pointer in vcc_cleanup

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

tty: vcc: hold port lock when clearing tty pointer in vcccleanup

vcccleanup() sets port->tty to NULL without holding port->lock, racing with vccevent() LDC callbacks that read port->tty under port->lock and then use tty->port. This can cause a use-after-free when the callback dereferences tty->port after cleanup has already destroyed and freed it.