CVE-2026-98752: tty: port: fix tty_port_tty_vhangup() race

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

tty: port: fix ttyportttyvhangup() race

Drivers rely on ttyportttyvhangup() to synchronously hang up their ports before tearing down resources as part of deregistration (e.g. on physical disconnect).

If ttyportttyvhangup() races with another hangup it may however return before the port has been shut down. This is in turn can lead to use-after-free and NULL-pointer dereferences in racing ioctls (including a synchronous hangup) and in processing of racing asynchronous hangups (carrier loss).

Add the missing serialisation to ttyportttyvhangup() and ttyporthangup() so that the former does not return until the port has been shut down also when there is a racing hangup.

Note that serial core does not use ttyporthangup(), but it already holds the port mutex when clearing port->tty and shutting down the port in uarthangup().