Rework set_header_word macros

Rework the set_header_word macros so that all compilers are
happy:
- the use of offsetof() is avoided, as it has an issue on IRIX
when the result is not constant
- the assignment within the macro is explicitely sequenced before
the function call to avoid a gcc sequence-point warning

Reviewed-on: http://gerrit.openafs.org/8816
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Simon Wilkinson <simonxwilkinson@gmail.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
(cherry picked from commit 7f96084969d3082842a9575ee8ff7da8c52a0796)

Change-Id: Iefdf7f6dbb60ffc569b67b3c89893fb43527eb02
Reviewed-on: http://gerrit.openafs.org/8818
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Simon Wilkinson <simonxwilkinson@gmail.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>