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)

(cherry picked from commit 5504bd59a39ba0bd0014308ff1aa5d4899432a3e)