net/pcap: fix byte stats for drop Tx

net/pcap: fix byte stats for drop Tx

Drop Tx path in pcap is Tx that just drops the packets, which is used
for the case only Rx from a pcap file is requested/matters.

The byte stats was calculated using first mbuf segment, which gives
wrong values for multi segmented mbufs, updated to use packet length
instead.

Bugzilla ID: 597
Fixes: a3f5252e5cbd ("net/pcap: enable infinitely Rx a pcap file")
Cc: stable@dpdk.org
Reported-by: Cian Ferriter <cian.ferriter@intel.com>
Signed-off-by: Ferruh Yigit <ferruh.yigit@intel.com>
Acked-by: Cian Ferriter <cian.ferriter@intel.com>