Merge branch 'ipv6-fix-two-gc-issues-with-permanent-routes'

Kuniyuki Iwashima says:

====================
ipv6: Fix two GC issues with permanent routes.

Patch 1 fixes the unbounded growth of tb6_gc_hlist due to
permanent routes whose exception routes have all expired.

Patch 2 fixes an issue where exception routes tied to
permanent routes are not properly aged.

Patch 3 is a selftest for the issue fixed by Patch 2.
====================

Link: https://patch.msgid.link/20260320072317.2561779-1-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>