netfilter: conntrack: warn when cleanup is stuck

nf_conntrack_cleanup_net_list() calls schedule() so it does not
show up as a hung task. Add an explicit check to make debugging
leaked skbs/conntack references more obvious.

Acked-by: Florian Westphal <fw@strlen.de>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20251207010942.1672972-5-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>