pg: incrementing len should be truncated by stream->max_packet_bytes
pg: incrementing len should be truncated by
stream->max_packet_bytes

This length is the payload length, and will impact the ip length
when ip length is not specified. iplen = header_len + payload_len
SO, better to make it comply with max_packet_bytes

Change-Id: I8b0f7485e29fcaccae656f2d03b3b5e614300fb8
Signed-off-by: Kingwel Xie <2a62226005ff76ec1b3ca9a6f915ba889ad44cc6@ericsson.com>

vpp-dev/vpp,chrisy/vpp,chrisy/vpp,chrisy/vpp,chrisy/vpp,FDio/vpp,FDio/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,vpp-dev/vpp,vpp-dev/vpp,FDio/vpp,vpp-dev/vpp,chrisy/vpp,chrisy/vpp,FDio/vpp,vpp-dev/vpp,chrisy/vpp,vpp-dev/vpp,FDio/vpp,FDio/vpp,vpp-dev/vpp