Disable router anycast address for /127 prefixes

RFC 6164 requires that routers MUST disable Subnet-Router anycast
for the prefix when /127 prefixes are used.

No need for matching code in addrconf_leave_anycast() as it
will silently ignore any attempt to leave an unknown anycast
address.
Signed-off-by: NBjørn Mork <bjorn@mork.no>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/addrconf.c

@@ -1470,6 +1470,8 @@ void addrconf_leave_solict(struct inet6_dev *idev, const struct in6_addr *addr)
 static void addrconf_join_anycast(struct inet6_ifaddr *ifp)
 {
 	struct in6_addr addr;
+	if (ifp->prefix_len == 127) /* RFC 6164 */
+		return;
 	ipv6_addr_prefix(&addr, &ifp->addr, ifp->prefix_len);
 	if (ipv6_addr_any(&addr))
 		return;