Rework the locking and lifecycle management in the cdc-acm driver.
Instead of using a global mutex to prevent the 'acm' object from being
freed, use the tty_port kref to keep the device alive when either the
USB side or TTY side is still active.

This allows us to use the global mutex purely for protecting the
acm_table, while use acm->mutex to guard against disconnect during
TTY port activation and shutdown.

The USB-side kref is taken during port initialization in probe(), and
released at the end of disconnect(). The TTY-side kref is taken in
install() and released in cleanup(). On disconnect, tty_vhangup() is
called instead of tty_hangup() to ensure the TTY hangup processing is
completed before the USB device is taken down.

The TTY open and close handlers have been gutted and replaced with
tty_port_open() and tty_port_close() respectively. The driver-specific
code which used to be there was spread across install(), activate() and
shutdown().

Reported-by: Dave Jones <davej@redhat.com>
Cc: Alan Cox <alan@linux.intel.com>
Cc: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Havard Skinnemoen <hskinnemoen@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>