net: pktgen: Remove unused 'allocated_skbs' field

Field pktgen_dev.allocated_skbs had been written to, but never read
from. The number of allocated skbs can be deduced anyway, from the total
number of sent packets and the 'clone_skb' param.

Signed-off-by: Bogdan Hamciuc <bogdan.hamciuc@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>