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 3985e8a3611a93bb36789f65db862e5700aab65e]

Bug: 19470192
Bug: 21832279
Bug: 22464419
Change-Id: I92e56d0db5b4945261ee0609d6dcac20376c4edf
Git-commit: 0633924e7776754ccb473d649e5fa9ee45617a46
Git-repo: https://android.googlesource.com/kernel/common/


Signed-off-by: Ashwanth Goli <ashwanth@codeaurora.org>