MFV r325609: 7531 Assign correct flags to prefetched buffers

illumos/illumos-gate@272952165423c254ad7708f1b3fe2ff0a6ce408b
https://github.com/illumos/illumos-gate/commit/272952165423c254ad7708f1b3fe2ff0a6ce408b

https://www.illumos.org/issues/7531

I found that some buffers that could be L2ARC eligible are not flagged
such, leading to some performance impact.  As a test I ran the same IO
workload 10 times in a raw.  It is a metadata only workload (files
listing).  l2arc_noprefetch=0.

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Prakash Surya <prakash.surya@delphix.com>
Approved by: Dan McDonald <danmcd@joyent.com>
Author: benrubson <ben.rubson@gmail.com>

MFC after: 8 days