ip6tnl: fix use after free of fb_tnl_dev

ip6tnl: fix use after free of fb_tnl_dev

The upstream commit bb8140947a24 ("ip6tnl: allow to use rtnl ops on fb tunnel")
(backported into linux-3.10.y) left a bug which was fixed upstream by commit
1e9f3d6f1c40 ("ip6tnl: fix use after free of fb_tnl_dev").

The problem is a bit different in linux-3.10.y, because there is no x-netns
support (upstream commit 0bd8762824e7 ("ip6tnl: add x-netns support")).
When ip6_tunnel.ko is unloaded, FB device is deleted by rtnl_link_unregister()
and then we try to delete it again in ip6_tnl_destroy_tunnels().

This patch removes the second deletion.

Reported-by: Steven Rostedt <rostedt@goodmis.org>
Suggested-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Cc: David Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>