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: I7ecb795e04a2bb3aae801a6feb57f205d3ad4d46
Reviewed-on: http://gerrit.openafs.org/10434
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>