staging: rtl8712: fix memory leak in rtl871x_load_fw_cb

staging: rtl8712: fix memory leak in rtl871x_load_fw_cb

There is a leak in rtl8712 driver.
The problem was in non-freed adapter data if
firmware load failed.

This leak can be reproduced with this code:
https://syzkaller.appspot.com/text?tag=ReproC&x=16612f02d00000,
Autoload must fail (to not hit memory leak reported by syzkaller)

There are 2 possible ways how rtl871x_load_fw_cb() and
r871xu_dev_remove() can be called (in case of fw load error).

1st case:
	r871xu_dev_remove() then rtl871x_load_fw_cb()

	In this case r871xu_dev_remove() will wait for
	completion and then will jump to the end, because
	rtl871x_load_fw_cb() set intfdata to NULL:

	if (pnetdev) {
		struct _adapter *padapter = netdev_priv(pnetdev);

		/* never exit with a firmware callback pending */
		wait_for_completion(&padapter->rtl8712_fw_ready);
		pnetdev = usb_get_intfdata(pusb_intf);
		usb_set_intfdata(pusb_intf, NULL);
		if (!pnetdev)
			goto firmware_load_fail;

		... clean up code here ...
	}

2nd case:
	rtl871x_load_fw_cb() then r871xu_dev_remove()

	In this case pnetdev (from code snippet above) will
	be zero (because rtl871x_load_fw_cb() set it to NULL)
	And clean up code won't be executed again.

So, in all cases we need to free adapted data in rtl871x_load_fw_cb(),
because disconnect function cannot take care of it. And there won't be
any race conditions, because complete() call happens after setting
intfdata to NULL.

In previous patch I moved out free_netdev() from r8712_free_drv_sw()
and that's why now it's possible to free adapter data and then call
complete.

Fixes: 8c213fa59199 ("staging: r8712u: Use asynchronous firmware loading")
Signed-off-by: Pavel Skripkin <paskripkin@gmail.com>
Link: https://lore.kernel.org/r/81e68fe0194499cc2e7692d35bc4dcf167827d8f.1623620630.git.paskripkin@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>