[NETNS]: Drop packets in the non-initial namespace on the per/protocol basis.

IP layer now can handle multiple namespaces normally. So, process such
packets normally and drop them only if the transport layer is not
aware about namespaces.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>