Do not touch internal workqueue. Call tty_vhangup instead.

Note that finished hangup does not necessarily mean that all processes
are dead. Especially when the tty is a console. The code assumes that
right now.
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Jiri Kosina <jkosina@suse.cz>
Acked-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>