usb: gadget: u_ether: Do not make UDC parent of the net device

The UDC is not a suitable parent of the net device as the UDC can
change or vanish during the lifecycle of the ethernet gadget. This
can be illustrated with the following:

mkdir -p /sys/kernel/config/usb_gadget/mygadget
cd /sys/kernel/config/usb_gadget/mygadget
mkdir -p configs/c.1/strings/0x409
echo "C1:Composite Device" > configs/c.1/strings/0x409/configuration
mkdir -p functions/ecm.usb0
ln -s functions/ecm.usb0 configs/c.1/
echo "dummy_udc.0" > UDC
rmmod dummy_hcd

The 'rmmod' removes the UDC from the just created gadget, leaving
the still existing net device with a no longer existing parent.

Accessing the ethernet device with commands like:

ip --details link show usb0

will result in a KASAN splat:

==================================================================
BUG: KASAN: use-after-free in if_nlmsg_size+0x3e8/0x528
Read of size 4 at addr c5c84754 by task ip/357

CPU: 3 PID: 357 Comm: ip Not tainted 6.1.0-rc3-00013-gd14953726b24-dirty #324
Hardware name: Freescale i.MX6 Quad/DualLite (Device Tree)
 unwind_backtrace from show_stack+0x10/0x14
 show_stack from dump_stack_lvl+0x58/0x70
 dump_stack_lvl from print_report+0x134/0x4d4
 print_report from kasan_report+0x78/0x10c
 kasan_report from if_nlmsg_size+0x3e8/0x528
 if_nlmsg_size from rtnl_getlink+0x2b4/0x4d0
 rtnl_getlink from rtnetlink_rcv_msg+0x1f4/0x674
 rtnetlink_rcv_msg from netlink_rcv_skb+0xb4/0x1f8
 netlink_rcv_skb from netlink_unicast+0x294/0x478
 netlink_unicast from netlink_sendmsg+0x328/0x640
 netlink_sendmsg from ____sys_sendmsg+0x2a4/0x3b4
 ____sys_sendmsg from ___sys_sendmsg+0xc8/0x12c
 ___sys_sendmsg from sys_sendmsg+0xa0/0x120
 sys_sendmsg from ret_fast_syscall+0x0/0x1c

Solve this by not setting the parent of the ethernet device.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Link: https://lore.kernel.org/r/20221104131031.850850-2-s.hauer@pengutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>