[ipv6] valid_lft and active connections

Hi,

I have the following problem:

If I use ipv6 addresses with valid_lft != forever, the ipv6 addresses
are removed from the interface if the valid_lft expires, even if there're
established connection which use with address.

Would it be possible keep the address until the last active connection
is closed? Otherwise the usable of the privacy extensions will make
very long living tcp connections impossible.

It would be also ok to add a new special value "used" similar to "forever",
which triggers the modified behavior.

metze