[NETNS]: Process netfilter hooks in initial namespace only.

There were no packets in the namespace other than initial
previously. This will be changed in the neareast future. Netfilters
are not namespace aware and should be processed in the initial
namespace only for now.

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