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Instead of only appending items to the list, removing them when the
netns has been deleted.

By doing that, we can make sure 'cleanup_all_ns()' is not trying to
remove already deleted netns.
Reviewed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://lore.kernel.org/r/20240607-upstream-net-next-20240607-selftests-mptcp-net-lib-v1-2-e36986faac94@kernel.org

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