net: contiki: Fix the RA ICMPv6 sending

The ICMPv6 RA 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: I3dc64cfce669de54283db290cfb009b2c6ea7225
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>