ip6mr: advertise new mfc entries via rtnl

This patch allows to monitor mf6c activities via rtnetlink.
To avoid parsing two times the mf6c oifs, we use maxvif to allocate the rtnl
msg, thus we may allocate some superfluous space.

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