Use offsetof() in set_header_word to get field offset

Use offsetof() in set_header_word to get field offset

Use offsetof() to replace a few instances where the same logic is
open coded in set_header_word and inc_header_word macros.  In cases
where the field name involves a variable as an index to an array,
newer gcc gives a sequence point warning.

Change-Id: I43e3d6ef6a63b51003496a1beb72c445a9109615
Reviewed-on: http://gerrit.openafs.org/6513
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>