selftests/bpf: Output the correct error code for pthread APIs

The return value of pthread API is the error code when the called
API fails, so output the return value instead of errno.

Signed-off-by: Hou Tao <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

tools/testing/selftests/bpf/bench.c

···
 static void set_thread_affinity(pthread_t thread, int cpu)
 {
 	cpu_set_t cpuset;
+	int err;
 
 	CPU_ZERO(&cpuset);
 	CPU_SET(cpu, &cpuset);
-	if (pthread_setaffinity_np(thread, sizeof(cpuset), &cpuset)) {
+	err = pthread_setaffinity_np(thread, sizeof(cpuset), &cpuset);
+	if (err) {
 		fprintf(stderr, "setting affinity to CPU #%d failed: %d\n",
-			cpu, errno);
+			cpu, -err);
 		exit(1);
 	}
 }
···
 				     bench->consumer_thread, (void *)(long)i);
 		if (err) {
 			fprintf(stderr, "failed to create consumer thread #%d: %d\n",
-				i, -errno);
+				i, -err);
 			exit(1);
 		}
 		if (env.affinity)
···
 				     bench->producer_thread, (void *)(long)i);
 		if (err) {
 			fprintf(stderr, "failed to create producer thread #%d: %d\n",
-				i, -errno);
+				i, -err);
 			exit(1);
 		}
 		if (env.affinity)