afs: consolidate disk block calculations

afs: consolidate disk block calculations

Similar code is used in several places to calculate the number of 1KiBi
blocks required for a cache chunk, rounded up to the nearest
afs_fsfragsize.  Coalesce these into a single inline function,
afs_round_to_fsfragsize.

No functional change should be incurred by this commit.

Change-Id: I1f58ad28426b786fc9809df01d409c8137437012
Reviewed-on: https://gerrit.openafs.org/14227
Reviewed-by: Cheyenne Wills <cwills@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>