selftests: nft_queue.sh: add a parallel stress test

Introduce a new stress test to check for race conditions in the
nfnetlink_queue subsystem, where an entry is freed while another CPU is
concurrently walking the global rhashtable.

To trigger this, `nf_queue.c` is extended with two new flags:
  * -O (out-of-order): Buffers packet IDs and flushes them in reverse.
  * -b (bogus verdicts): Floods the kernel with non-existent packet IDs.

The bogus verdict loop forces the kernel's lookup function to perform
full rhashtable bucket traversals (-ENOENT). Combined with reverse-order
flushing and heavy parallel UDP/ping flooding across 8 queues, this puts
the nfnetlink_queue code under pressure.

Joint work with Florian Westphal.

Signed-off-by: Fernando Fernandez Mancera <fmancera@suse.de>
Signed-off-by: Florian Westphal <fw@strlen.de>

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