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.

CRs-fixed: 1009682
Change-Id: Ia7de62a93bed3baed596bc16ed471f2c73ed513f
Signed-off-by: Erik Kline <ek@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Git-commit: dbd204200c68d8bd4a6561cf471df5293546441a
Git-repo: https://android.googlesource.com/kernel/common/


[subashab@codeaurora.org: resolve trivial merge conflicts]
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>