net/pcap: capture only ingress packets from Rx iface

net/pcap: capture only ingress packets from Rx iface

Support rx of in direction packets only
Useful for apps that also tx to eth_pcap ports in order to not see them
echoed back in as rx when out direction is also captured

Example:
In case using rx_iface and sending *single* packet to eth1
it will loop forever as the when it is sent to tx_iface=eth1
it will be captured again on the rx_iface=eth1 and so on
  $RTE_TARGET/app/testpmd l 0-3 -n 4 \
	--vdev 'net_pcap0,rx_iface=eth1,tx_iface=eth1'
  …
  ---------------------- Forward statistics for port 0  ------------
  RX-packets: 758            RX-dropped: 0             RX-total: 758
  TX-packets: 758            TX-dropped: 0             TX-total: 758
  ------------------------------------------------------------------
While if using rx_iface_in it will not be captured on the way out and
be forwarded only once
  $RTE_TARGET/app/testpmd l 0-3 -n 4 \
	--vdev 'net_pcap0,rx_iface_in=eth1,tx_iface=eth1'
  …
  ---------------------- Forward statistics for port 0  ------------
  RX-packets: 1              RX-dropped: 0             RX-total: 1
  TX-packets: 1              TX-dropped: 0             TX-total: 1
  ------------------------------------------------------------------

Signed-off-by: Ido Goshen <ido@cgstowernetworks.com>
Acked-by: Ferruh Yigit <ferruh.yigit@intel.com>