7003 zap_lockdir() should tag hold

zap_lockdir() / zap_unlockdir() should take a "void *tag" argument which
tags the hold on the zap. This will help diagnose programming errors
which misuse the hold on the ZAP.

Sponsored by: Intel Corp.

Closes #108

Reviewed by: Pavel Zakharov <pavel.zakharov@delphix.com>
Reviewed by: Steve Gonczi <steve.gonczi@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Author: Matthew Ahrens <mahrens@delphix.com>

openzfs/openzfs@0780b3eab5a2c13e04328b39ecd2a6d0d3c4f7cb