Write issue taskq shouldn't be dynamic

This is as much an upstream compatibility as it's a bit of a performance
gain.

The illumos taskq implemention doesn't allow a TASKQ_THREADS_CPU_PCT type
to be dynamic and in fact enforces as much with an ASSERT.

As to performance, if this taskq is dynamic, it can cause excessive
contention on tq_lock as the threads are created and destroyed because it
can see bursts of many thousands of tasks in a short time, particularly
in heavy high-concurrency zvol write workloads.

Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Tim Chase <[email protected]>
Closes #5236 

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