When the leaf_rcv union was introduced to support ktls RX offload, it was put at
the front of the pkthhdr, and pushed the commonly accessed csum_flags and
rsstype into the next cacheline.

Given that csum_flags is accessed on every pkt sent and every pkt freed,
but the leaf_rcv fields are only used for ktls rx offload, it makes more sense
to have csum_flags in a cacheline that is already likely to be hot.