staging: udlfb: revamp reference handling to insure successful shutdown

staging: udlfb: revamp reference handling to insure successful shutdown

Revamp reference handling and synchronization for unload/shutdown

Udlfb is a "virtual" framebuffer device that really exists on
two separate stacks: at the bottom of the framebuffer interface,
and on top of USB.  During unload, there's no guarantee which
one will tear down first. So reference counting must be solid
to handle all possibilities and not access anything once its gone.

Signed-off-by: Bernie Thompson <bernie@plugable.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>