net: ipv6: allow choosing optimistic addresses with use_optimistic

The use_optimistic sysctl makes optimistic IPv6 addresses
equivalent to preferred addresses for source address selection
(e.g., when calling connect()), but it does not allow an
application to bind to optimistic addresses. This behaviour is
inconsistent - for example, it doesn't make sense for bind() to
an optimistic address fail with EADDRNOTAVAIL, but connect() to
choose that address outgoing address on the same socket.

Bug: 17769720
Bug: 18609055
Change-Id: Ia0ed70a3698fdc78ffa2a50113872b4b3ee66450
Signed-off-by: Erik Kline <ek@google.com>
Signed-off-by: Lorenzo Colitti <lorenzo@google.com>
Git-commit: 0065bf4206cd4fecb482aef751fd39360fa8bafa
Git-repo: https://android.googlesource.com/kernel/common/


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