Fixes a IPv6 UDP fragmentation regression introduced in v6.6.93.
Fixes: https://github.com/openwrt/openwrt/issues/19198
Link: https://github.com/openwrt/openwrt/pull/19199
(cherry picked from commit f62c33bc4a)
Link: https://github.com/openwrt/openwrt/pull/19201
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>