batman-adv: Avoid double-rtnl_lock ELP metric worker

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

batman-adv: Avoid double-rtnl_lock ELP metric worker

batadv_v_elp_get_throughput() might be called when the RTNL lock is already

held. This could be problematic when the work queue item is cancelled via

cancel_delayed_work_sync() in batadv_v_elp_iface_disable(). In this case,

an rtnl_lock() would cause a deadlock.

To avoid this, rtnl_trylock() was used in this function to skip the

retrieval of the ethtool information in case the RTNL lock was already

held.

But for cfg80211 interfaces, batadv_get_real_netdev() was called - which

also uses rtnl_lock(). The approach for __ethtool_get_link_ksettings() must

also be used instead and the lockless version __batadv_get_real_netdev()

has to be called.