afs: afs_CacheIsTooFull macro refs wrong constant

When afs_CacheIsTooFull tests the number of free blocks it should
use CM_DCACHESPACEFREEPCT (90%) instead of CM_DCACHECOUNTFREEPCT (95%).

Change-Id: I1c9ac4f9a6d03077047837af6ef9a09e256ea07d
Reviewed-on: http://gerrit.openafs.org/10235
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>