ipv6 route: Fix route lifetime in netlink message.

1) We may have route lifetime larger than INT_MAX.
In that case we had wired value in lifetime.
Use INT_MAX if lifetime does not fit in s32.

2) Lifetime is valid iif RTF_EXPIRES is set.

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>