[IPV6]: Defer IPv6 device initialization until the link becomes ready.

NETDEV_UP might be sent even if the link attached to the interface was
not ready.  DAD does not make sense in such case, so we won't do so.
After interface

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
diff --git a/include/net/if_inet6.h b/include/net/if_inet6.h
index e97a9ac..d8234f9 100644
--- a/include/net/if_inet6.h
+++ b/include/net/if_inet6.h
@@ -24,6 +24,7 @@
 #define IF_RA_MANAGED	0x40
 #define IF_RA_RCVD	0x20
 #define IF_RS_SENT	0x10
+#define IF_READY	0x80000000
 
 /* prefix flags */
 #define IF_PREFIX_ONLINK	0x01