afs: FreeDCache test afs_blocksDiscarded

For consistency with afs_FlushDCache and afs_DiscardDCache
include afs_blocksDiscarded in the free space test.  When afs_FreeDCache
is called it should be zero.

Change-Id: I84a5dea626258c628d24bf647088e2cbfd233c9a
Reviewed-on: http://gerrit.openafs.org/10431
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>