geneve: move debug check after netdev unregister

The debug check must be done after unregister_netdevice_many() call --
the list_del() for this is done inside .ndo_stop.

Fixes: 2843a253

 ("geneve: speedup geneve tunnels dismantle")
Reported-and-tested-by:  <syzbot+68a8ed58e3d17c700de5@syzkaller.appspotmail.com>
Cc: Haishuang Yan <yanhaishuang@cmss.chinamobile.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>