network: fix SIGABRT related to unreachable route with DHCP6

After #17834, unreachable routes generated through DHCP6 are managed by
Manager. But they are referrenced by the DHCP6 uplink. So, the routes
managed by Manager must be freed after all Link objects are freed.

Follow-up for 575f14ee.

Fixes SIGABRT reproted in #17831.