afs: Fix some dcache-related comments

 - The comments preceding the afs_CacheIsTooFull macro, describing the
   cache-related high and low water marks, are a little out of date.
   We start freeing on 90% space, not 95%, and we also can take into
   account how many free/used chunks we have.

 - afs_WakeCacheWaitersIfDrained looks at the number of non-used (free
   or discarded) blocks, not just free blocks.

Change-Id: I5bf923b3c24993294e752f4312737815d0102779
Reviewed-on: http://gerrit.openafs.org/10459
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>