net/9p/usbg: Add new usb gadget function transport

net/9p/usbg: Add new usb gadget function transport

Add the new gadget function for 9pfs transport. This function is
defining an simple 9pfs transport interface that consists of one in and
one out endpoint. The endpoints transmit and receive the 9pfs protocol
payload when mounting a 9p filesystem over usb.

Tested-by: Andrzej Pietrasiewicz <andrzej.p@collabora.com>
Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
Link: https://lore.kernel.org/r/20240116-ml-topic-u9p-v12-2-9a27de5160e0@pengutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>