selftests/bpf: Ensure that next_cpu() returns a valid CPU number

selftests/bpf: Ensure that next_cpu() returns a valid CPU number

When using option -a without --prod-affinity or --cons-affinity, if the
number of producers and consumers is greater than the number of online
CPUs, the benchmark will fail to run as shown below:

  $ getconf _NPROCESSORS_ONLN
  8
  $ ./bench bpf-loop -a -p9
  Setting up benchmark 'bpf-loop'...
  setting affinity to CPU #8 failed: -22

Fix it by returning the remainder of next_cpu divided by the number of
online CPUs in next_cpu().

Signed-off-by: Hou Tao <houtao1@huawei.com>
Link: https://lore.kernel.org/r/20230613080921.1623219-4-houtao@huaweicloud.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>