ipv6: sysctl to restrict candidate source addresses

Per RFC 6724, section 4, "Candidate Source Addresses":

    It is RECOMMENDED that the candidate source addresses be the set
    of unicast addresses assigned to the interface that will be used
    to send to the destination (the "outgoing" interface).

Add a sysctl to enable this behaviour.

Signed-off-by: Erik Kline <ek@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt
index f63aeef..1a5ab21b 100644
--- a/Documentation/networking/ip-sysctl.txt
+++ b/Documentation/networking/ip-sysctl.txt
@@ -1460,6 +1460,13 @@
 	routers are present.
 	Default: 3
 
+use_oif_addrs_only - BOOLEAN
+	When enabled, the candidate source addresses for destinations
+	routed via this interface are restricted to the set of addresses
+	configured on this interface (vis. RFC 6724, section 4).
+
+	Default: false
+
 use_tempaddr - INTEGER
 	Preference for Privacy Extensions (RFC3041).
 	  <= 0 : disable Privacy Extensions