net: ipv6: Use net_nbuf_unref() to get better debug prints

When going through fragments in update_ll_reserve(), use
net_nbuf_unref() in order to get better memory allocation
debug prints.

Change-Id: Ia7ee15b32800acdebeb28125465515132c55e179
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>