ip6mr: fix rtm_family of rtnl msg

We talk about IPv6, hence the family is RTNL_FAMILY_IP6MR!
rtnl_register() is already called with RTNL_FAMILY_IP6MR.

The bug is here since the beginning of this function (commit 5b285cac).

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>