selftests/bpf: Stress LRU rqspinlock recovery paths

Introduces stress test for bpf_lru_list that exercises
lock-failures and orphan-recovery, added by the LRU rqspinlock
conversion.

Runs three subtests: common LRU, per-CPU LRU lists (BPF_F_NO_COMMON_LRU),
and per-CPU LRU map. Each pins one userspace hammer per CPU and attaches
the perf_event NMI BPF prog (update+delete mix) on every online CPU.
Pre-fix, lockdep fires the "INITIAL USE -> IN-NMI" splat during stress.

After stress test, drain_then_verify_capacity() drains every key
and refills the lru map.
A stranded node on any CPU's pool would have forced eviction of
a just-inserted key on that CPU, surfacing here as a missing lookup.

Marked serial_ because per-CPU pinning and high-rate HW perf events
would perturb parallel tests.

Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
Link: https://lore.kernel.org/r/20260607-lru_map_spin-v3-3-bcd9332e911b@meta.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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