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: Ic9063280d88eb28d84851cbe6b7e4867a1110659
Reviewed-on: http://gerrit.openafs.org/10232
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
afs: Introduce afs_WakeCacheWaitersIfDrained

Consolidate common code into afs_WakeCacheWaitersIfDrained().

Change-Id: I10e35afbc1f1469038e111a7bbd209c8897c8972
Reviewed-on: http://gerrit.openafs.org/10233
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>