Preliminary work to make IPv6 multicast forwarding netns-aware.

Declare IPv6 multicast forwarding variables 'mroute_do_assert' and
'mroute_do_pim' per-namespace in struct netns_ipv6.

At the moment, these variables are only referenced in init_net.
Signed-off-by: Benjamin Thery <benjamin.thery@bull.net>
Signed-off-by: David S. Miller <davem@davemloft.net>