ipv6: only save scopeid when it is required

The kernel only requires that we supply a scope id to bind() when the
source address is of certain types, so lets only save/supply the scope id
in these cases.

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>