ipv6: route: per route IP tunnel metadata via lightweight tunnel

Allow specification of per route IP tunnel instructions also for IPv6.
This complements commit 3093fbe7 ("route: Per route IP tunnel metadata
via lightweight tunnel").

Signed-off-by: Jiri Benc <jbenc@redhat.com>
CC: YOSHIFUJI Hideaki <hideaki.yoshifuji@miraclelinux.com>
Acked-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>