app/testpmd: extend forwarding statistics to 64 bits

app/testpmd: extend forwarding statistics to 64 bits

fwd engine statistics are stored as unsigned int (32bits) and can wrap
quite quickly.
Example: sending 7mpps for 614s gives us 4298000000 packets =>
0x1002e4680 larger than 32bits.

testpmd reports forwarding stats as:
RX-packets: 3500381        TX-packets: 3500010        TX-dropped: 371

While the port and accumulated stats are reported as 64bits:
RX-packets: 4298467677     RX-dropped: 0             RX-total: 4298467677
TX-packets: 4298467306     TX-dropped: 371           TX-total: 4298467677

Fixes: af75078fece3 ("first public release")
Cc: stable@dpdk.org
Signed-off-by: David Marchand <david.marchand@redhat.com>
Reviewed-by: Andrew Rybchenko <arybchenko@solarflare.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>