ipv6: send unsolicited neighbour advertisements to all-nodes

[ Upstream commit 60713a0ca7fd6651b951cc1b4dbd528d1fc0281b ]

As documented in RFC4861 (Neighbor Discovery for IP version 6) 7.2.6.,
unsolicited neighbour advertisements should be sent to the all-nodes
multicast address.

Change-Id: I03105d31b7ef064e3b2487dd994eea3bab273543
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>