ax25_addr_size
int ax25_addr_size(const ax25_digi *);
skb_push(skb, 1 + ax25_addr_size(dp));
skb_pull(skb, ax25_addr_size(&dp));
headroom = ax25_addr_size(ax25->digipeat);
dptr = skb_push(skb, ax25_addr_size(digi));