net: wan: lapbether.c: Use built-in RCU list checking

The only callers of the function lapbeth_get_x25_dev()
are lapbeth_rcv() and lapbeth_device_event().

lapbeth_rcv() uses rcu_read_lock() whereas lapbeth_device_event()
is called with RTNL held (As mentioned in the comments).

Therefore, pass lockdep_rtnl_is_held() as cond argument in
list_for_each_entry_rcu();

Signed-off-by: Madhuparna Bhowmik <madhuparnabhowmik04@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>