ipv6: Clamp reported valid_lft to a minimum of 0

Since addresses are only revalidated every 2 minutes, the reported
valid_lft can underflow shortly before the address is deleted.
Clamp it to a minimum of 0, as for prefered_lft.

Reported-by: Piotr Lewandowski <piotr.lewandowski@gmail.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>