macsec: sync features on RTM_NEWLINK

Syzkaller managed to lock the lower device via ETHTOOL_SFEATURES:

 netdev_lock include/linux/netdevice.h:2761 [inline]
 netdev_lock_ops include/net/netdev_lock.h:42 [inline]
 netdev_sync_lower_features net/core/dev.c:10649 [inline]
 __netdev_update_features+0xcb1/0x1be0 net/core/dev.c:10819
 netdev_update_features+0x6d/0xe0 net/core/dev.c:10876
 macsec_notify+0x2f5/0x660 drivers/net/macsec.c:4533
 notifier_call_chain+0x1b3/0x3e0 kernel/notifier.c:85
 call_netdevice_notifiers_extack net/core/dev.c:2267 [inline]
 call_netdevice_notifiers net/core/dev.c:2281 [inline]
 netdev_features_change+0x85/0xc0 net/core/dev.c:1570
 __dev_ethtool net/ethtool/ioctl.c:3469 [inline]
 dev_ethtool+0x1536/0x19b0 net/ethtool/ioctl.c:3502
 dev_ioctl+0x392/0x1150 net/core/dev_ioctl.c:759

It happens because lower features are out of sync with the upper:

  __dev_ethtool (real_dev)
    netdev_lock_ops(real_dev)
    ETHTOOL_SFEATURES
      __netdev_features_change
        netdev_sync_upper_features
          disable LRO on the lower
    if (old_features != dev->features)
      netdev_features_change
        fires NETDEV_FEAT_CHANGE
	macsec_notify
	  NETDEV_FEAT_CHANGE
	    netdev_update_features (for each macsec dev)
	      netdev_sync_lower_features
	        if (upper_features != lower_features)
	          netdev_lock_ops(lower) # lower == real_dev
		  stuck
		  ...

    netdev_unlock_ops(real_dev)

Per commit af5f54b ("net: Lock lower level devices when updating
features"), we elide the lock/unlock when the upper and lower features
are synced. Makes sure the lower (real_dev) has proper features after
the macsec link has been created. This makes sure we never hit the
situation where we need to sync upper flags to the lower.

Reported-by: syzbot+7e0f89fb6cae5d002de0@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=7e0f89fb6cae5d002de0
Fixes: 7e4d784 ("net: hold netdev instance lock during rtnetlink operations")
Signed-off-by: Stanislav Fomichev <sdf@fomichev.me>
Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Link: https://patch.msgid.link/20250908173614.3358264-1-sdf@fomichev.me
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: Stanislav Fomichev

drivers/net/macsec.c

@@ -4286,6 +4286,7 @@ static int macsec_newlink(struct net_device *dev,
 	if (err < 0)
 		goto del_dev;
 
+	netdev_update_features(dev);
 	netif_stacked_transfer_operstate(real_dev, dev);
 	linkwatch_fire_event(dev);