tty: Make vt's have a tty_port

The vt layer isn't safely handling reference counts to tty object on the input
side. Add a tty port structure to the vt layer in order to implement this using
the standard helpers.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>