pktgen: support net namespace

v3: make pktgen_threads list per-namespace
v2: remove a useless check

This patch add net namespace to pktgen, so that
we can use pktgen in different namespaces.

Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Cong Wang <amwang@redhat.com>
Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>