net: nbuf: Do not delete the fragments in net_nbuf_unref()

Show only what would be freed when debugging the allocations.
Let the net_buf_unref() to delete the individual fragments.

Change-Id: I1a39a553f12d73b78c4ba08be0e8e8252b666c8e
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>