shared: add nm_utils_getpagesize() and use it in netlink code

Since we already cached the result of getpagesize() in a static variable (at
two places), move the code to nm-shared-utils, so it is reusable.

Also, use sysconf() instead of getpagesize(), like suggested by `man
getpagesize`.

shared/nm-utils/nm-shared-utils.c

@@ -2285,3 +2285,35 @@ nm_utils_invoke_on_idle (NMUtilsInvokeOnIdleCallback callback,
 		data->cancelled_id = 0;
 	data->idle_id = g_idle_add (_nm_utils_invoke_on_idle_cb_idle, data);
 }
+
+/*****************************************************************************/
+
+int
+nm_utils_getpagesize (void)
+{
+	static volatile int val = 0;
+	long l;
+	int v;
+
+	v = g_atomic_int_get (&val);
+
+	if (G_UNLIKELY (v == 0)) {
+		l = sysconf (_SC_PAGESIZE);
+
+		g_return_val_if_fail (l > 0 && l < G_MAXINT, 4*1024);
+
+		v = (int) l;
+		if (!g_atomic_int_compare_and_exchange (&val, 0, v)) {
+			v = g_atomic_int_get (&val);
+			g_return_val_if_fail (v > 0, 4*1024);
+		}
+	}
+
+	nm_assert (v > 0);
+#if NM_MORE_ASSERTS > 5
+	nm_assert (v == getpagesize ());
+	nm_assert (v == sysconf (_SC_PAGESIZE));
+#endif
+
+	return v;
+}