ipv6 addrconf: Allow infinite prefix lifetime.

We need to handle infinite prefix lifetime specially.
With help from original reporter "Bonitch, Joseph"
<Joseph.Bonitch@xerox.com>.

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>