net: nbuf: Print more info when trying double free net_buf

Print information who freed the net_buf last time if this
can be figured out. This info is not fully accurate if there
is lot of allocations going on but could give some indicatation
who was using the net_buf previously.

Change-Id: I3d4463c54a9b944847a532d951fb6d2303fc06cf
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>