vol: move ih_PkgDefaultsSet check inside ih_PkgDefaults

vol: move ih_PkgDefaultsSet check inside ih_PkgDefaults

Instead of repeating the oneshot check in each caller of ih_PkgDefaults,
move the oneshot check into ih_PkgDefaults itself.

While here, ensure that ih_PkgDefaults does its work under IH_LOCK.

Finally, make ih_PkgDefaultsSet a local static variable (no longer
exported).

Change-Id: I66717de12cb5cc70de0507a768f968f6afbd38f8
Reviewed-on: https://gerrit.openafs.org/14383
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>