selftests/bpf: skip the timer_lockup test for single-CPU nodes

selftests/bpf: skip the timer_lockup test for single-CPU nodes

The timer_lockup test needs 2 CPUs to work, on single-CPU nodes it fails
to set thread affinity to CPU 1 since it doesn't exist:

    # ./test_progs -t timer_lockup
    test_timer_lockup:PASS:timer_lockup__open_and_load 0 nsec
    test_timer_lockup:PASS:pthread_create thread1 0 nsec
    test_timer_lockup:PASS:pthread_create thread2 0 nsec
    timer_lockup_thread:PASS:cpu affinity 0 nsec
    timer_lockup_thread:FAIL:cpu affinity unexpected error: 22 (errno 0)
    test_timer_lockup:PASS: 0 nsec
    #406     timer_lockup:FAIL

Skip the test if only 1 CPU is available.

Signed-off-by: Viktor Malik <vmalik@redhat.com>
Fixes: 50bd5a0c658d1 ("selftests/bpf: Add timer lockup selftest")
Tested-by: Philo Lu <lulie@linux.alibaba.com>
Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20241107115231.75200-1-vmalik@redhat.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>