ipv6: drop packets when source address is multicast

RFC 4291 Section 2.7 says Multicast addresses must not be used as source
addresses in IPv6 packets - drop them on input so we don't process the
packet further.

Signed-off-by: Brian Haley <brian.haley@hp.com>
Reported-and-Tested-by: Kumar Sanghvi <divinekumar@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>