ipv6: Don't add routes to ipv6 disabled interfaces.

If the interface has IPv6 disabled, don't add a multicast or
link-local route since we won't be adding a link-local address.

Reported-by: Mahesh Kelkar <maheshkelkar@gmail.com>
Signed-off-by: Brian Haley <brian.haley@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>