MFC r284593: MFV r284412: 5911 ZFS "hangs" while deleting file

illumos/illumos-gate@46e1baa6cf6d5432f5fd231bb588df8f9570c858
https://www.illumos.org/issues/5911
Sometimes ZFS appears to hang while deleting a file. It is actually
making slow progress at the file deletion, but other operations
(administrative and writes via the data path) "hang" until the file
removal completes, which can take a long time if the file has many
blocks. The deletion (or most of it) happens in a single txg, and the
sync thread spends most of its time reading indirect blocks...

Reviewed by: Bayard Bell <buffer.g.overflow@gmail.com>
Reviewed by: Alek Pinchuk <alek@nexenta.com>
Reviewed by: Simon Klinkert <simon.klinkert@gmail.com>
Reviewed by: Dan McDonald <danmcd@omniti.com>
Approved by: Richard Lowe <richlowe@richlowe.net>
Author: Matthew Ahrens <mahrens@delphix.com>

PR: 199775