netifd: Prevent flapping IPv6 routes

netifd: Prevent flapping IPv6 routes

Comparing valid_until will always return false as the value is updated for each
route update message. This causes IPv6 routes to jump more around than House of
Pain, which might have undesirable consequences for user-space and user-space
applications.

Removing the valid_until comparison when setting keep fixes this problem, and
seems to have no side-effects. I am no IPv6 expert, but I see that valid of the
route is updated correctly and route is deleted if I block the route update
messages.

Signed-off-by: Kristian Evensen <kristian.evensen@gmail.com>