net: ipv6: make "ip -6 route get mark xyz" work.

Currently, "ip -6 route get mark xyz" ignores the mark passed in
by userspace. Make it honour the mark, just like IPv4 does.

[net-next commit 2e47b291953c35afa4e20a65475954c1a1b9afe1]

Change-Id: Ief6cd1e1b7e43dc0d008b7e692be62cadc5cc7ca
Signed-off-by: Lorenzo Colitti <lorenzo@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>