net: use .data.once section in netdev_level_once()

Same rationale than prior patch : using the dedicated
section avoid holes and pack all these bool values.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/linux/netdevice.h

@@ -4942,7 +4942,7 @@ void netdev_info(const struct net_device *dev, const char *format, ...);
 
 #define netdev_level_once(level, dev, fmt, ...)			\
 do {								\
-	static bool __print_once __read_mostly;			\
+	static bool __section(".data.once") __print_once;	\
 								\
 	if (!__print_once) {					\
 		__print_once = true;				\