usbnet: Prevents free active kevent

The root cause of this issue are:
1. When probing the usbnet device, executing usbnet_link_change(dev, 0, 0);
put the kevent work in global workqueue. However, the kevent has not yet
been scheduled when the usbnet device is unregistered. Therefore, executing
free_netdev() results in the "free active object (kevent)" error reported
here.

2. Another factor is that when calling usbnet_disconnect()->unregister_netdev(),
if the usbnet device is up, ndo_stop() is executed to cancel the kevent.
However, because the device is not up, ndo_stop() is not executed.

The solution to this problem is to cancel the kevent before executing
free_netdev().

Fixes: a69e617e533e ("usbnet: Fix linkwatch use-after-free on disconnect")
Reported-by: Sam Sun <samsun1006219@gmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=8bfd7bcc98f7300afb84
Signed-off-by: Lizhi Xu <lizhi.xu@windriver.com>
Link: https://patch.msgid.link/20251022024007.1831898-1-lizhi.xu@windriver.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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