usb: gadget: u_serial: Fix race condition in TTY wakeup

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

usb: gadget: u_serial: Fix race condition in TTY wakeup

A race condition occurs when gs_start_io() calls either gs_start_rx() or gs_start_tx(), as those functions briefly drop the port_lock for usb_ep_queue(). This allows gs_close() and gserial_disconnect() to clear port.tty and port_usb, respectively.

Use the null-safe TTY Port helper function to wake up TTY.

Example CPU1: CPU2: gserial_connect() // lock gs_close() // await lock gs_start_rx() // unlock usb_ep_queue() gs_close() // lock, reset port.tty and unlock gs_start_rx() // lock tty_wakeup() // NPE