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;
+}

shared/nm-utils/nm-shared-utils.h

@@ -1086,4 +1086,8 @@ nm_strv_ptrarray_take_gstring (GPtrArray *cmd,
 	                                                 FALSE));
 }
 
+/*****************************************************************************/
+
+int nm_utils_getpagesize (void);
+
 #endif /* __NM_SHARED_UTILS_H__ */

src/platform/nm-netlink.c

@@ -255,20 +255,6 @@ nla_reserve (struct nl_msg *msg, int attrtype, int attrlen)
 
 /*****************************************************************************/
 
-static int
-get_default_page_size (void)
-{
-	static int val = 0;
-	int v;
-
-	if (G_UNLIKELY (val == 0)) {
-		v = getpagesize ();
-		g_assert (v > 0);
-		val = v;
-	}
-	return val;
-}
-
 struct nl_msg *
 nlmsg_alloc_size (size_t len)
 {
@@ -298,7 +284,7 @@ nlmsg_alloc_size (size_t len)
 struct nl_msg *
 nlmsg_alloc (void)
 {
-	return nlmsg_alloc_size (get_default_page_size ());
+	return nlmsg_alloc_size (nm_utils_getpagesize ());
 }
 
 struct nl_msg *
@@ -1334,7 +1320,6 @@ nl_recv (struct nl_sock *sk, struct sockaddr_nl *nla,
 {
 	ssize_t n;
 	int flags = 0;
-	static int page_size = 0;
 	struct iovec iov;
 	struct msghdr msg = {
 		.msg_name = (void *) nla,
@@ -1354,10 +1339,8 @@ nl_recv (struct nl_sock *sk, struct sockaddr_nl *nla,
 	        && sk->s_bufsize == 0))
 		flags |= MSG_PEEK | MSG_TRUNC;
 
-	if (page_size == 0)
+	iov.iov_len =    sk->s_bufsize
-		page_size = getpagesize () * 4;
+	              ?: (((size_t) nm_utils_getpagesize ()) * 4u);
-
-	iov.iov_len = sk->s_bufsize ?: page_size;
 	iov.iov_base = g_malloc (iov.iov_len);
 
 	if (   creds