router: always check ra_default

router: always check ra_default

We currently only check ra_default when an interface has valid
addresses. This results in ra_default being ignored in case we have an
interface with only link-local addresses. This effectively breaks the
use of value 2 for the ra_default parameter.

Fix this by always checking ra_lifetime, regardless of the interface
having public addresses or not.

Fixes: #11930
Fixes: 83e14f455817 ("router: advertise removed addresses as invalid in 3 consecutive RAs")
Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>
Acked-by:Hans Dedecker <dedeckeh@gmail.com>