ipmr/ip6mr: advertise mfc stats via rtnetlink

These statistics can be checked only via /proc/net/ip_mr_cache or
SIOCGETSGCNT[_IN6] and thus only for the table RT_TABLE_DEFAULT.
Advertising them via rtnetlink allows to get statistics for all cache entries,
whatever the table is.

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