MFC r344441: Fix few issues in ioat(4) driver.

- Do not explicitly count active descriptors. It allows hardware reset
to happen while device is still referenced, plus simplifies locking.
- Do not

MFC r344441: Fix few issues in ioat(4) driver.

- Do not explicitly count active descriptors. It allows hardware reset
to happen while device is still referenced, plus simplifies locking.
- Do not stop/start callout each time the queue becomes empty. Let it
run to completion and rearm if needed, that is much cheaper then to touch
it every time, plus also simplifies locking.
- Decouple submit and cleanup locks, making driver reentrant.
- Avoid memory mapped status register read on every interrupt.
- Improve locking during device attach/detach.
- Remove some no longer used variables.

Sponsored by: iXsystems, Inc.

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MFC r343125 (by cem): ioat(4): Set __result_use_check on ioat_acquire_reserve

Even M_WAITOK callers must check for failure. For example, if the device is
quiescing, either due to automatic error-re

MFC r343125 (by cem): ioat(4): Set __result_use_check on ioat_acquire_reserve

Even M_WAITOK callers must check for failure. For example, if the device is
quiescing, either due to automatic error-recovery induced reset, or due to
administrative detach, the routine will return ENXIO and the acquire
reference will not be held. So, there is no mode in which it is safe to
assume the routine succeeds without checking.

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