afs: define NUMPAGGROUPS once

afs: define NUMPAGGROUPS once

Define the number of groups per PAG in one place.  Prefix the define
with AFS_ to avoid name conflicts in the future (unlikely as it may be).

Fix the misnamed AFSPAGGGROUPS symbol in linux implementation of two
groups per PAG.

Change-Id: I78bb42913f2a5d84c9f323f17dc36d800d8acb84
Reviewed-on: https://gerrit.openafs.org/12382
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>