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>

[Simplified back-port of net-next 3985e8a3]

Bug: 19470192
Bug: 21832279
Bug: 22464419
Change-Id: Ib74ef945dcabe64215064f15ee1660b6524d65ce