pppoe: RCU locking in get_item_by_addr()

Use dev_get_by_name_rcu() instead of dev_get_by_name(),
to avoid touching device refcount in hotpath.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>