net: nbuf: Fix debug prints in memory pool init and unref

Wrong pool pointer was used when printing pool information during
the unref in nbuf.c. During nbuf init, wrong pool pointer was used
when printing memory pool sizes.

Change-Id: I9ed08cf7afa3c841c97ea981b97fff37aa40a984
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>