OpenZFS 9328 - zap code can take advantage of c99

OpenZFS 9328 - zap code can take advantage of c99

The ZAP code was written before we allowed c99 in the Solaris kernel. We
should change it to take advantage of being able to declare variables where
they are first used. This reduces variable scope and means less scrolling
to find the type of variables.

Authored by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Steve Gonczi <steve.gonczi@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Approved by: Dan McDonald <danmcd@joyent.com>
Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>
OpenZFS-issue: https://illumos.org/issues/9328
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/76ead05
Closes #7578