netfilter: nfnetlink_queue: optimize verdict lookup with hash table

The current implementation uses a linear list to find queued packets by
ID when processing verdicts from userspace. With large queue depths and
out-of-order verdicting, this O(n) lookup becomes a significant
bottleneck, causing userspace verdict processing to dominate CPU time.

Replace the linear search with a hash table for O(1) average-case
packet lookup by ID. A global rhashtable spanning all network
namespaces attributes hash bucket memory to kernel but is subject to
fixed upper bound.

Signed-off-by: Scott Mitchell <scott.k.mitch1@gmail.com>
Signed-off-by: Florian Westphal <fw@strlen.de>

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