net: rtnetlink: use netif_oper_up instead of open code

The open code is defined as a new helper function(netif_oper_up) on netdev.h,
the code is dev->operstate == IF_OPER_UP || dev->operstate == IF_OPER_UNKNOWN.
Thus, replace the open code to netif_oper_up. This patch doesn't change logic.
Signed-off-by: Juhee Kang <claudiajkang@gmail.com>
Link: https://lore.kernel.org/r/20220831125845.1333-1-claudiajkang@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/core/rtnetlink.c

@@ -866,14 +866,12 @@ static void set_operstate(struct net_device *dev, unsigned char transition)
 		break;
 
 	case IF_OPER_TESTING:
-		if (operstate == IF_OPER_UP ||
-		    operstate == IF_OPER_UNKNOWN)
+		if (netif_oper_up(dev))
 			operstate = IF_OPER_TESTING;
 		break;
 
 	case IF_OPER_DORMANT:
-		if (operstate == IF_OPER_UP ||
-		    operstate == IF_OPER_UNKNOWN)
+		if (netif_oper_up(dev))
 			operstate = IF_OPER_DORMANT;
 		break;
 	}