vti6: fix uninit when using x-netns

When the kernel deleted a vti6 interface, this interface was not removed from
the tunnels list. Thus, when the ip6_vti module was removed, this old interface
was found and the kernel tried to delete it again. This was leading to a kernel
panic.

Fixes: 61220ab3

 ("vti6: Enable namespace changing")
Signed-off-by: Yao Xiwei <xiwei.yao@6wind.com>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>