afs: Consolidate fheader initialization

We were initializing an afs_fheader structure in two different places,
with the same values. Consolidate these into a single function, so
updating the structure is easier. Also zero the whole structure, just
to make sure everything is initialized, even if the structure changes.

This commit should have no behavior impact; it is just code
reorganization.

Change-Id: If90757166d8490eaa053aa086c7b95349a62332e
Reviewed-on: http://gerrit.openafs.org/11510
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Perry Ruiter <pruiter@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>