selftests/bpf: Fix race condition in enable_stats

In parallel execution mode, this test now need to use atomic operation
to avoid race condition.

Signed-off-by: Yucong Sun <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/testing/selftests/bpf/progs/test_enable_stats.c

···
 SEC("raw_tracepoint/sys_enter")
 int test_enable_stats(void *ctx)
 {
-	count += 1;
+	__sync_fetch_and_add(&count, 1);
 	return 0;
 }