ipv6 addrconf: Implemented enhanced DAD (RFC7527)

Implemented RFC7527 Enhanced DAD.
IPv6 duplicate address detection can fail if there is some temporary
loopback of Ethernet frames. RFC7527 solves this by including a random
nonce in the NS messages used for DAD, and if an NS is received with the
same nonce it is assumed to be a looped back DAD probe and is ignored.
RFC7527 is enabled by default. Can be disabled by setting both of
conf/{all,interface}/enhanced_dad to zero.

Signed-off-by: Erik Nordmark <nordmark@arista.com>
Signed-off-by: Bob Gilligan <gilligan@arista.com>
Reviewed-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/ipv6.h

@@ -68,6 +68,7 @@ struct ipv6_devconf {
 #ifdef CONFIG_IPV6_SEG6_HMAC
 	__s32		seg6_require_hmac;
 #endif
+	__u32		enhanced_dad;
 
 	struct ctl_table_header *sysctl_header;
 };