It appears the kernel does not send notifications via netlink if the
default route is removed in some cases.  This causes the platform
route cache to become stale, and thus when the default route is
reset by NM the platform thinks the route already exists, and does
not add it.  But the route doesn't exist, becuase the kernel silently
removed it without telling anyone.

Fix that with a big hammer by flushing/refilling the route cache when
devices are deactivated (deletion of their addresses causes the default
route to be removed by the kernel) and when the default route is
updated by NM itself.

Pavel: if we find a more granular method, we should probably revert
this as the cache refill can be expensive.