Preserve packet order between tcp_lro_rx() and if_input() to avoid
creating extra corner cases. If no packets can be LROed, combine them
into one chain for submission via if_input(). If any packet can
potentially be LROed however, retain old behaviour and call if_input()
for each packet.
This should keep the 12% improvement for small packet forwarding intact,
but mostly avoids impacting the LRO case.