Prevent deadlock in cad_down()

Prevent deadlock in cad_down()

Introduce an rwlock to serialize cad(4) ioctl operations so that
cad_down() can release NET_LOCK() temporarily when invoking barriers.
This releasing prevents a possible deadlock with the taskq barrier.
The deadlock was pointed out by witness(4).

In addition, release NET_LOCK() when allocating memory in cad_up()
to reduce the risk of ill effects.