selftests/bpf: Fix arena_spin_lock on systems with less than 16 CPUs

selftests/bpf: Fix arena_spin_lock on systems with less than 16 CPUs

test_arena_spin_lock_size() explicitly requires having at least 2 CPUs,
but if the machine has less than 16, then pthread_setaffinity_np() call
in spin_lock_thread() fails.

Cap threads to the number of CPUs.

Alternative solutions are raising the number of required CPUs to 16, or
pinning multiple threads to the same CPU, but they are not that useful.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Link: https://lore.kernel.org/r/20250424165525.154403-3-iii@linux.ibm.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>