slip: stop double free sl->dev in slip_open

BugLink: https://bugs.launchpad.net/bugs/1868628

After include 3b5a3997 ("slip: Fix memory leak in slip_open error path")
and e58c1912 ("slip: Fix use-after-free Read in slip_open") with 4.4.y/4.9.y.
We will trigger a bug since we can double free sl->dev in slip_open. Actually,
we should backport cf124db5 ("net: Fix inconsistent teardown and release
of private netdev state.") too since it has delete free_netdev from sl_free_netdev.
Fix it by delete free_netdev from slip_open.
Signed-off-by: yangerkun <yangerkun@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>
Signed-off-by: Kelsey Skunberg <kelsey.skunberg@canonical.com>

drivers/net/slip/slip.c

@@ -868,7 +868,6 @@ static int slip_open(struct tty_struct *tty)
 	tty->disc_data = NULL;
 	clear_bit(SLF_INUSE, &sl->flags);
 	sl_free_netdev(sl->dev);
-	free_netdev(sl->dev);
 
 err_exit:
 	rtnl_unlock();