OpenZFS 7545 - zdb should disable reference tracking

Authored by: Matthew Ahrens <mahrens@delphix.com>
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>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Ported-by: Giuseppe Di Natale <dinatale2@llnl.gov>

Porting Notes: Moved reference_tracking_enable and
reference_history outside of ZFS_DEBUG.

OpenZFS-issue: https://www.illumos.org/issues/7545
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/4dd77f9
Closes #5701