The slot reserve step handed any free slot to whoever asked first, so
I/O could fill the queue and leave no slot for the driver's own admin
commands, which a reset sends through the same queue. A full queue was
also reported to CAM as busy, and CAM used up its retries on that
within microseconds.
Keep the last slot for admin requests and give CAM one opening fewer,
since letting CAM fill every I/O slot cut the throughput sharply. On a
full queue, freeze the device queue and requeue the request. Remove the
unused reserve_admin_slot hook.
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