serial: core: fix transmit-buffer reset and memleak

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

serial: core: fix transmit-buffer reset and memleak

Commit 761ed4a94582 ("tty: serial_core: convert uart_close to use

tty_port_close") converted serial core to use tty_port_close() but

failed to notice that the transmit buffer still needs to be freed on

final close.

Not freeing the transmit buffer means that the buffer is no longer

cleared on next open so that any ioctl() waiting for the buffer to drain

might wait indefinitely (e.g. on termios changes) or that stale data can

end up being transmitted in case tx is restarted.

Furthermore, the buffer of any port that has been opened would leak on

driver unbind.

Note that the port lock is held when clearing the buffer pointer due to

the ldisc race worked around by commit a5ba1d95e46e ("uart: fix race

between uart_put_char() and uart_shutdown()").

Also note that the tty-port shutdown() callback is not called for

console ports so it is not strictly necessary to free the buffer page

after releasing the lock (cf. d72402145ace ("tty/serial: do not free

trasnmit buffer page under port lock")).