[PKTGEN]: Fix Initialization fail leak.

Even if pktgen's thread initialization fails for all CPUs, the module
will be successfully loaded.

This patch changes that behaivor, by returning an error on module load time,
and also freeing all the resources allocated. It also prints a warning if a
thread initialization has failed.

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