net/net_pkt: Fully separate struct net_pkt from struct net_buf

- net_pkt becomes a stand-alone structure with network packet meta
  information.
- network packet data is still managed through net_buf, mostly named
  'frag'.
- net_pkt memory management is done through k_mem_slab
- function got introduced or relevantly renamed to target eithe net_pkt
  or net_buf fragments.
- net_buf's sent_list ends up in net_pkt now, and thus helps to save
  memory when TCP is enabled.

Change-Id: Ibd5c17df4f75891dec79db723a4c9fc704eb843d
Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>