mm: add sysfs entry for pmadv_ksm syscall number

As `pmadv_ksm()` syscall is out-of-tree, maintain a separate
`/sys/kernel/pmadv/ksm` file with a syscall number so that userspace
tools can always know what to use in runtime even if the syscall number
shifts after adding another in-tree syscall.

The idea is borrowed from futex2 submission.

Signed-off-by: Oleksandr Natalenko <oleksandr@natalenko.name>

mm/madvise.c

@@ -1614,3 +1614,28 @@ SYSCALL_DEFINE3(pmadv_ksm, int, pidfd, int, behaviour, unsigned int, flags)
 	return -ENOSYS;
 #endif /* CONFIG_KSM */
 }
+
+#ifdef CONFIG_KSM
+static ssize_t ksm_show(struct kobject *kobj, struct kobj_attribute *attr,
+			 char *buf)
+{
+	return sprintf(buf, "%u\n", __NR_pmadv_ksm);
+}
+static struct kobj_attribute pmadv_ksm_attr = __ATTR_RO(ksm);
+
+static struct attribute *pmadv_sysfs_attrs[] = {
+	&pmadv_ksm_attr.attr,
+	NULL,
+};
+
+static const struct attribute_group pmadv_sysfs_attr_group = {
+	.attrs = pmadv_sysfs_attrs,
+	.name = "pmadv",
+};
+
+static int __init pmadv_sysfs_init(void)
+{
+	return sysfs_create_group(kernel_kobj, &pmadv_sysfs_attr_group);
+}
+subsys_initcall(pmadv_sysfs_init);
+#endif /* CONFIG_KSM */