net: contiki: Fix the RS ICMPv6 sending

The ICMPv6 RS 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: Iddedb7b6674f30c7943c0ba59c0afc7d244a1e0d
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>