net/ipv4: fix IPv4 multicast over network namespaces

When using multicast over a local bridge feeding a number of LXC guests
using veth, the LXC guests are unable to get a response from other guests
when pinging 224.0.0.1.  Multicast packets did not appear to be getting
delivered to the network namespaces of the guest hosts, and further
inspection showed that the incoming route was pointing to the loopback
device of the host, not the guest.  This lead to the wrong network namespace
being picked up by sockets (like ICMP).  Fix this by using the correct
network namespace when creating the inbound route entry.

Signed-off-by: Benjamin LaHaise <bcrl@kvack.org>
Signed-off-by: David S. Miller <davem@davemloft.net>