OpenZFS 2976 - remove useless offsetof() macros

OpenZFS 2976 - remove useless offsetof() macros

Authored by: Richard Lowe <richlowe@richlowe.net>
Reviewed by: Josef 'Jeff' Sipek <jeffpc@josefsipek.net>
Reviewed by: Igor Kozhukhov <ikozhukhov@gmail.com>
Reviewed by: Andy Stormont <andyjstormont@gmail.com>
Approved by: Dan McDonald <danmcd@omniti.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Ported-by: Giuseppe Di Natale <dinatale2@llnl.gov>
OpenZFS-issue: https://www.illumos.org/issues/2976
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/5c5f137
Closes #6582