ipv6: simplify detection of first operational link-local address on interface

In commit 1ec047eb ("ipv6: introduce per-interface counter for
dad-completed ipv6 addresses") I build the detection of the first
operational link-local address much to complex. Additionally this code
now has a race condition.

Replace it with a much simpler variant, which just scans the address
list when duplicate address detection completes, to check if this is
the first valid link local address and send RS and MLD reports then.

Fixes: 1ec047eb

 ("ipv6: introduce per-interface counter for dad-completed ipv6 addresses")
Reported-by: Jiri Pirko <jiri@resnulli.us>
Cc: Flavio Leitner <fbl@redhat.com>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Acked-by: Flavio Leitner <fbl@redhat.com>
Acked-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>