afs: afs_CacheTruncateDaemon wake waiters !too full

When processing afs_CacheTruncateDaemon() if the cache is no longer
too full, then wake the waiters.

Change-Id: I3dfc2251baad984bd53c4ddaa38f03293eb3ec1c
Reviewed-on: http://gerrit.openafs.org/10435
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>