Illumos #734: Use taskq_dispatch_ent() interface

It has been observed that some of the hottest locks are those
of the zio taskqs.  Contention on these locks can limit the
rate at which zios are dispatched which limits performance.

This upstream change from Illumos uses new interface to the
taskqs which allow them to utilize a prealloc'ed taskq_ent_t.
This removes the need to perform an allocation at dispatch
time while holding the contended lock.  This has the effect
of improving system performance.

Reviewed by: Albert Lee <[email protected]>
Reviewed by: Richard Lowe <[email protected]>
Reviewed by: Alexey Zaytsev <[email protected]>
Reviewed by: Jason Brian King <[email protected]>
Reviewed by: George Wilson <[email protected]>
Reviewed by: Adam Leventhal <[email protected]>
Approved by: Gordon Ross <[email protected]>

References to Illumos issue:
  https://www.illumos.org/issues/734

Ported-by: Prakash Surya <[email protected]>
Signed-off-by: Brian Behlendorf <[email protected]>
Closes #482

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