1. Add local_df field to struct sk_buff to mark packets which
   are to be fragmented locally despite of their IPv6ness of IP DF flag
2. Add ext2_header_len to tcp_opt to keep memory of part of header length
   depending on route
3. Add trailer_len to struct dst_entry and xfrm_state to know how
   much of space should be reserved at tail of frame for subsequent
   transformations.
4. [BUG] icv_trun_len must be used while mss claculation, not
   icv_full_length.