MFV r316860: 7545 zdb should disable reference tracking

MFV r316860: 7545 zdb should disable reference tracking

illumos/illumos-gate@4dd77f9e38ef05b39db128ff7608d926fd3218c6
https://github.com/illumos/illumos-gate/commit/4dd77f9e38ef05b39db128ff7608d926fd3218c6

https://www.illumos.org/issues/7545
  When evicting from the ARC, we manipulate some refcount_t's, e.g. arcs_size.
  When using zdb to examine a large amount of data (e.g. zdb -bb on a large pool
  with small blocks), the ARC may have a large number of entries. If reference
  tracking is enabled, there will be ~1 reference for each block in the ARC. When
  evicting, we decrement the refcount and have to search all the references to
  find the one that we are removing, which is very slow.
  Since zdb is typically used to find problems with the on-disk format, and not
  with the code it is running, we should disable reference tracking in zdb.

Reviewed by: Dan Kimmel <dan.kimmel@delphix.com>
Reviewed by: Steve Gonczi <steve.gonczi@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Matthew Ahrens <mahrens@delphix.com>

MFC after:	2 weeks