locking: Change locking order, move NAL after instances

Some opcodes, for example LUInstanceFailover and LUInstanceMigrate,
can't know whether they need to acquire all nodes until they have a lock
on the instance. In turn they would have to acquire the node allocation
lock “just in case” and could only release it once the instance had been
acquired.

Since there is no good reason for the node allocation lock to be before
instances in the locking order, this patch changes the order so that
instances are locked first.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>