[PKTGEN]: Ports thread list to Kernel list implementation.

The final result is a simpler and smaller code.

Note that I'm adding a new member in the struct pktgen_thread called
'removed'. The reason is that I didn't find a better wait condition to
be used in the place of the replaced one.

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