Zebra: Handle IPv6 address status during initialization

Zebra: Handle IPv6 address status during initialization

At init time, Zebra queries the kernel for all interfaces. At this time,
an IPv6 address may exist on an interface but IPv6 DAD has not completed,
or it could be that DAD has failed. Zebra should examine the flags on
the address and act accordingly. Otherwise, it may end up with addresses
and routes which are not actually valid in the kernel, and this may lead
to, for example, BGP attempting neighbor connections on an interface on
which the source IPv6 address is not yet valid.

Signed-off-by: Vivek Venkatraman <vivek@cumulusnetworks.com>
Reviewed-by:   Dinesh Dutt <ddutt@cumulusnetworks.com>
Ticket: CM-7176
Reviewed By: CCR-3815
Testing Done: Verify failed test (needs temporary test change)

Note: Imported from 2.5-br patch zebra-handle-ipv6-addr-status.patch.