ipv6: clean up routes when manually removing address with a lifetime

ipv6: clean up routes when manually removing address with a lifetime

When an IPv6 address with a finite lifetime (configured with valid_lft
and preferred_lft) is manually deleted, the kernel does not clean up the
associated prefix route. This results in orphaned routes (marked "proto
kernel") remaining in the routing table even after their corresponding
address has been deleted.

This is particularly problematic on networks using combination of SLAAC
and bridges.

1. Machine comes up and performs RA on eth0.
2. User creates a bridge
   - does an ip -6 addr flush dev eth0;
   - adds the eth0 under the bridge.
3. SLAAC happens on br0.

Even tho the address has "moved" to br0 there will still be a route
pointing to eth0, but eth0 is not usable for IP any more.

Reviewed-by: David Ahern <dsahern@kernel.org>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20251113031700.3736285-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>