OpenZFS 7531 - Assign correct flags to prefetched buffers

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Prakash Surya <prakash.surya@delphix.com>
Authored by: abraunegg <alex.braunegg@gmail.com>
Approved by: Dan McDonald <danmcd@joyent.com>
Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>

OpenZFS-issue: https://www.illumos.org/issues/7531
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/468008cb