LinuxKPI: SKB update
- skb_reset_tail_pointer(): we do not do offsets so do a plain reset
- skb_add_rx_frag(): adjust data_len to keep track of the frag
- based on that implement skb_is_nonlinear() and skb_linearize()
- implement build_skb() and adjust linuxkpi_kfree_skb() and ddb macro.
Sponsored by: The FreeBSD Foundation (partially)
MFC after: 3 days