net: nbuf: Let's remove inner type

It's possible to know from which pool the nbuf comes from so
let's reduce nbuf size by removing its internal type attribute which
becomes then useless.

When a data buffer is not coming from nbuf data buffer pool, let's call
it "EXTERNAL", just to make debugging handling a little simple in the
code.

Change-Id: I6931394c8c4f594137f6380be0a0ba5cea371040
Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>