USB: cdc-acm: Fix disconnect() vs close() race

USB: cdc-acm: Fix disconnect() vs close() race

There's a race between the USB disconnect handler and the TTY close
handler which may cause the acm object to be freed while it's still
being used. This may lead to things like

http://article.gmane.org/gmane.linux.usb.general/54250

and

https://lkml.org/lkml/2011/5/29/64

This is the simplest fix I could come up with. Holding on to open_mutex
while closing the TTY device prevents acm_disconnect() from freeing the
acm object between acm->port.count drops to 0 and the TTY side of the
cleanups are finalized.

Signed-off-by: Havard Skinnemoen <hskinnemoen@google.com>
Cc: Oliver Neukum <oliver@neukum.name>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>