[PKTGEN]: Ports if_list to the in-kernel implementation.

This patch ports the per-thread interface list list to the in-kernel
linked list implementation. In the general, the resulting code is a
bit simpler.

Signed-off-by: Luiz Capitulino <lcapitulino@mandriva.com.br>
Signed-off-by: David S. Miller <davem@davemloft.net>