From RFC 3493, Section 5.2:

       IPV6_MULTICAST_IF

          Set the interface to use for outgoing multicast packets.  The
          argument is the index of the interface to use.  If the
          interface index is specified as zero, the system selects the
          interface (for example, by looking up the address in a routing
          table and using the resulting interface).

This patch adds support for (index == 0) to reset the value to it's 
original state, allowing the system to choose the best interface.  IPv4 
already behaves this way.
Signed-off-by: Brian Haley <brian.haley@hp.com>
Acked-by: David L Stevens <dlstevens@us.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>