Hi,
please consider reverting
commit 84379c9afe011020e797e3f50a662b08a6355dcf netfilter: ipv6: nf_defrag: drop skb dst before queueing
It causes kernel crash for locally generated ipv6 fragments when netfilter ipv6 defragmentation is used.
The faulty commit is not essential for -stable, it only delays netns teardown for longer than needed when that netns still has ipv6 frags queued. Much better than crash :-/
commit ids are: 4.4.y: not affected (not backported) 4.9.y: backported as ad8b1ffc3efae2f65080bdb11145c87d299b8f9a 4.14.y: backported as 28c74ff85efd192aeca9005499ca50c24d795f61 4.18.y: (first affected kernel): 84379c9afe011020e797e3f50a662b08a6355dcf
For 4.19.y, you could also wait for a bug fix to hit Linus tree, I can ping you again once its in: https://patchwork.ozlabs.org/patch/988233/
Thanks, Florian