net, ip6_tunnel: fix namespaces move

in the same manner as commit d0f41851 ("net, ip_tunnel: fix
namespaces move"), fix namespace moving as it was broken since commit
8d79266b ("ip6_tunnel: add collect_md mode to IPv6 tunnel"), but for
ipv6 this time; there is no reason to keep it for ip6_tunnel.

Fixes: 8d79266b ("ip6_tunnel: add collect_md mode to IPv6 tunnel")
Signed-off-by: William Dauchy <w.dauchy@criteo.com>
Acked-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>