nm_txsync_prologue
if (nm_txsync_prologue(kring, ring) >= kring->nkr_num_slots) {
if (nm_txsync_prologue(kring, ring) >= kring->nkr_num_slots) {
uint32_t nm_txsync_prologue(struct netmap_kring *, struct netmap_ring *);
if (unlikely(nm_txsync_prologue(kring, &shadow_ring) >= num_slots)) {