sched/psi: create /proc/pressure and /proc/pressure/{io|memory|cpu} only when psi enabled

when CONFIG_PSI_DEFAULT_DISABLED set to N or the command line set psi=0,
I think we should not create /proc/pressure and
/proc/pressure/{io|memory|cpu}.

In the future, user maybe determine whether the psi feature is enabled by
checking the existence of the /proc/pressure dir or
/proc/pressure/{io|memory|cpu} files.
Signed-off-by: Wang Long <w@laoqinren.net>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Link: https://lkml.kernel.org/r/1576672698-32504-1-git-send-email-w@laoqinren.net

kernel/sched/psi.c

@@ -1280,10 +1280,12 @@ static const struct file_operations psi_cpu_fops = {
 
 static int __init psi_proc_init(void)
 {
+	if (psi_enable) {
 		proc_mkdir("pressure", NULL);
 		proc_create("pressure/io", 0, NULL, &psi_io_fops);
 		proc_create("pressure/memory", 0, NULL, &psi_memory_fops);
 		proc_create("pressure/cpu", 0, NULL, &psi_cpu_fops);
+	}
 	return 0;
 }
 module_init(psi_proc_init);