[IPV6] ADDRCONF: Optimistic Duplicate Address Detection (RFC 4429) Support.

Nominally an autoconfigured IPv6 address is added to an interface in the
Tentative state (as per RFC 2462).  Addresses in this state remain in this
state while the Duplicate Address Detection process operates on them to
determine their uniqueness on the network.  During this period, these
tentative addresses may not be used for communication, increasing the time
before a node may be able to communicate on a network.  Using Optimistic
Duplicate Address Detection, autoconfigured addresses may be used
immediately for communication on the network, as long as certain rules are
followed to avoid conflicts with other nodes during the Duplicate Address
Detection process.

Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/linux/if_addr.h b/include/linux/if_addr.h
index d557e4c..43f3bed 100644
--- a/include/linux/if_addr.h
+++ b/include/linux/if_addr.h
@@ -39,6 +39,7 @@
 #define IFA_F_TEMPORARY		IFA_F_SECONDARY
 
 #define	IFA_F_NODAD		0x02
+#define IFA_F_OPTIMISTIC	0x04
 #define	IFA_F_HOMEADDRESS	0x10
 #define IFA_F_DEPRECATED	0x20
 #define IFA_F_TENTATIVE		0x40