USB: serial: fix lifetime and locking problems

USB: serial: fix lifetime and locking problems

This is commit 2d93148ab6988cad872e65d694c95e8944e1b626 back-ported to
2.6.29.

This patch (as1229-3) fixes a few lifetime and locking problems in the
usb-serial driver.  The main symptom is that an invalid kevent is
created when the serial device is unplugged while a connection is
active.

	Ports should be unregistered when device is disconnected,
	not when the parent usb_serial structure is deallocated.

	Each open file should hold a reference to the corresponding
	port structure, and the reference should be released when
	the file is closed.

	serial->disc_mutex should be acquired in serial_open(), to
	resolve the classic race between open and disconnect.

	serial_close() doesn't need to hold both serial->disc_mutex
	and port->mutex at the same time.

	Release the subdriver's module reference only after releasing
	all the other references, in case one of the release routines
	needs to invoke some code in the subdriver module.

	Replace a call to flush_scheduled_work() (which is prone to
	deadlocks) with cancel_work_sync().  Also, add a call to
	cancel_work_sync() in the disconnect routine.

	Reduce the scope of serial->disc_mutex in serial_disconnect().
	The only place it really needs to protect is where the
	"disconnected" flag is set.

	Call the shutdown method from within serial_disconnect()
	instead of destroy_serial(), because some subdrivers expect
	the port data structures still to be in existence when
	their shutdown method runs.

This fixes the bug reported in

	http://bugs.freedesktop.org/show_bug.cgi?id=20703

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>