USB: ftdi_sio: fix use-after-free in TIOCMIWAIT

USB: ftdi_sio: fix use-after-free in TIOCMIWAIT

commit 71ccb9b01981fabae27d3c98260ea4613207618e upstream.

Use the port wait queue and make sure to check the serial disconnected
flag before accessing private port data after waking up.

This is is needed as the private port data (including the wait queue
itself) can be gone when waking up after a disconnect.

When switching to tty ports, some lifetime assumptions were changed.
Specifically, close can now be called before the final tty reference is
dropped as part of hangup at device disconnect. Even with the ftdi
private-data refcounting this means that the port private data can be
freed while a process is sleeping on modem-status changes and thus
cannot be relied on to detect disconnects when woken up.

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>