netns: ip6mr: store netns in struct mfc6_cache

This patch stores into struct mfc6_cache the network namespace each
mfc6_cache belongs to. The new member is mfc6_net.

mfc6_net is assigned at cache allocation and doesn't change during
the rest of the cache entry life.

This will help to retrieve the current netns around the IPv6 multicast
forwarding code.

At the moment, all mfc6_cache are allocated in init_net.

Changelog:
==========
* Use write_pnet()/read_pnet() to set and get mfc6_net.

Signed-off-by: Benjamin Thery <benjamin.thery@bull.net>
Signed-off-by: David S. Miller <davem@davemloft.net>