net: switchdev: rename switchdev_lower_dev_find to switchdev_lower_dev_find_rcu

switchdev_lower_dev_find() assumes RCU read-side critical section
calling context, since it uses netdev_walk_all_lower_dev_rcu().

Rename it appropriately, in preparation of adding a similar iterator
that assumes writer-side rtnl_mutex protection.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>