netns: ipmr: store netns in struct mfc_cache

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

mfc_net is assigned at cache allocation and doesn't change during
the rest of the cache entry life.
A new net parameter is added to ipmr_cache_alloc/ipmr_cache_alloc_unres.

This will help to retrieve the current netns around the IPv4 multicast
routing code.

At the moment, all mfc_cache are allocated in init_net.

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