MFV 286550: 5694 traverse_prefetcher does not prefetch enough

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Alex Reece <alex@delphix.com>
Reviewed by: Christopher Siden <christopher.siden@delphix.com>
Reviewed by: Josef 'Jeff' Sipek <josef.sipek@nexenta.com>
Reviewed by: Bayard Bell <buffer.g.overflow@gmail.com>
Approved by: Garrett D'Amore <garrett@damore.org>
Author: George Wilson <george.wilson@delphix.com>

illumos/illumos-gate@34d7ce052c4565b078f73b95ccbd49274e98edaa