The correct lock order for LCU lock and cdev lock is to take the cdev
lock first and afterwards the LCU lock. This is caused by the fact
that LCU functions are called in an interrupt context with the cdev
lock implicitly hold by CIO.

To assure the right locking order but also be able to iterate over
devices in a LCU introduce a trylock block that can be called with
the device lock for one device hold and then takes the LCU lock and
try to lock all devices accounted to this LCU. Afterwards all devices
and the LCU itself are locked.
Signed-off-by: Stefan Haberland <stefan.haberland@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>