Illumos 6281 - prefetching should apply to 1MB reads

Illumos 6281 - prefetching should apply to 1MB reads

6281 prefetching should apply to 1MB reads
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Paul Dagnelie <pcd@delphix.com>
Reviewed by: Alexander Motin <mav@freebsd.org>
Reviewed by: Dan McDonald <danmcd@omniti.com>
Reviewed by: Justin Gibbs <gibbs@scsiguy.com>
Reviewed by: Xin Li <delphij@freebsd.org>
Approved by: Gordon Ross <gordon.ross@nexenta.com>

References:
  https://www.illumos.org/issues/6281
  https://github.com/illumos/illumos-gate/commit/6328027

Ported-by: kernelOfTruth kerneloftruth@gmail.com
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>