vxlan: fix nonfunctional neigh_reduce()

The VXLAN neigh_reduce() code is completely non-functional since
check-in. Specific errors:

1) The original code drops all packets with a multicast destination address,
	even though neighbor solicitations are sent to the solicited-node
	address, a multicast address. The code after this check was never run.
2) The neighbor table lookup used the IPv6 header destination, which is the
	solicited node address, rather than the target address from the
	neighbor solicitation. So neighbor lookups would always fail if it
	got this far. Also for L3MISSes.
3) The code calls ndisc_send_na(), which does a send on the tunnel device.
	The context for neigh_reduce() is the transmit path, vxlan_xmit(),
	where the host or a bridge-attached neighbor is trying to transmit
	a neighbor solicitation. To respond to it, the tunnel endpoint needs
	to do a *receive* of the appropriate neighbor advertisement. Doing a
	send, would only try to send the advertisement, encapsulated, to the
	remote destinations in the fdb -- hosts that definitely did not do the
	corresponding solicitation.
4) The code uses the tunnel endpoint IPv6 forwarding flag to determine the
	isrouter flag in the advertisement. This has nothing to do with whether
	or not the target is a router, and generally won't be set since the
	tunnel endpoint is bridging, not routing, traffic.

	The patch below creates a proxy neighbor advertisement to respond to
neighbor solicitions as intended, providing proper IPv6 support for neighbor
reduction.

Signed-off-by: David L Stevens <dlstevens@us.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>