ospf6d: fix freebsd mcast group issues

ospf6d: fix freebsd mcast group issues

There's a delay in FreeBSD between issuing a command to leave a
multicast group and an actual leave. If we execute "no router ospf6" and
"router ospf6" fast enough, we can end up in a situation when OS
performs the leave later than it performs the join and the interface
remains without a multicast group.

Instead of counting on a one second delay, we must wait until the
interface actually leaves the group.

Signed-off-by: Igor Ryzhov <iryzhov@nfware.com>