macsec: sync features on RTM_NEWLINK

In the Linux kernel, the following vulnerability has been resolved:

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 af5f54b0ef9e ("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.