net: contiki: Fix the NS ICMPv6 sending

The ICMPv6 NS payload was ignored and not sent. This was seen
as malformed packet by receiver. We must update the length of
the message by calling net_buf_add().

Change-Id: I5bf53695c811c27a75320fea7331d91782a9748a
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>