usb: gadget: f_ncm: align net_device lifecycle with bind/unbind

usb: gadget: f_ncm: align net_device lifecycle with bind/unbind

Currently, the net_device is allocated in ncm_alloc_inst() and freed in
ncm_free_inst(). This ties the network interface's lifetime to the
configuration instance rather than the USB connection (bind/unbind).

This decoupling causes issues when the USB gadget is disconnected where
the underlying gadget device is removed. The net_device can outlive its
parent, leading to dangling sysfs links and NULL pointer dereferences
when accessing the freed gadget device.

Problem 1: NULL pointer dereference on disconnect
 Unable to handle kernel NULL pointer dereference at virtual address
 0000000000000000
 Call trace:
   __pi_strlen+0x14/0x150
   rtnl_fill_ifinfo+0x6b4/0x708
   rtmsg_ifinfo_build_skb+0xd8/0x13c
   rtmsg_ifinfo+0x50/0xa0
   __dev_notify_flags+0x4c/0x1f0
   dev_change_flags+0x54/0x70
   do_setlink+0x390/0xebc
   rtnl_newlink+0x7d0/0xac8
   rtnetlink_rcv_msg+0x27c/0x410
   netlink_rcv_skb+0x134/0x150
   rtnetlink_rcv+0x18/0x28
   netlink_unicast+0x254/0x3f0
   netlink_sendmsg+0x2e0/0x3d4

Problem 2: Dangling sysfs symlinks
 console:/ # ls -l /sys/class/net/ncm0
 lrwxrwxrwx ... /sys/class/net/ncm0 ->
 /sys/devices/platform/.../gadget.0/net/ncm0
 console:/ # ls -l /sys/devices/platform/.../gadget.0/net/ncm0
 ls: .../gadget.0/net/ncm0: No such file or directory

Move the net_device allocation to ncm_bind() and deallocation to
ncm_unbind(). This ensures the network interface exists only when the
gadget function is actually bound to a configuration.

To support pre-bind configuration (e.g., setting interface name or MAC
address via configfs), cache user-provided options in f_ncm_opts
using the gether_opts structure. Apply these cached settings to the
net_device upon creation in ncm_bind().

Preserve the use-after-free fix from commit 6334b8e4553c ("usb: gadget:
f_ncm: Fix UAF ncm object at re-bind after usb ep transport error").
Check opts->net in ncm_set_alt() and ncm_disable() to ensure
gether_disconnect() runs only if a connection was established.

Fixes: 40d133d7f542 ("usb: gadget: f_ncm: convert to new function interface with backward compatibility")
Cc: stable@kernel.org
Signed-off-by: Kuen-Han Tsai <khtsai@google.com>
Link: https://patch.msgid.link/20251230-ncm-refactor-v1-3-793e347bc7a7@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>