ipv6: add exception routes to GC list in rt6_insert_exception

Commit 5eb902b8 ("net/ipv6: Remove expired routes with a separated list
of routes.") introduced a separated list for managing route expiration via
the GC timer.

However, it missed adding exception routes (created by ip6_rt_update_pmtu()
and rt6_do_redirect()) to this GC list. As a result, these exceptions were
never considered for expiration and removal, leading to stale entries
persisting in the routing table.

This patch fixes the issue by calling fib6_add_gc_list() in
rt6_insert_exception(), ensuring that exception routes are properly tracked
and garbage collected when expired.

Fixes: 5eb902b8 ("net/ipv6: Remove expired routes with a separated list of routes.")
Reported-by: Jianlin Shi <jishi@redhat.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://patch.msgid.link/837e7506ffb63f47faa2b05d9b85481aad28e1a4.1744134377.git.lucien.xin@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>