pktgen: tx_bytes might be slightly wrong

cur_pkt_size can be changed in proc fs while pktgen is running,
we better use a private field to get precise tx-bytes counter.

Signed-off-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>