afs: CacheTruncateDaemon work until Cache Drained

The afs_CacheTruncateDeamon() thread will not sleep until both
'afs_CacheTooFull' and 'afs_WaitForCacheDrain' are true but the
thread will stop freeing space in the cache when 'afs_CacheTooFull'
is true which prevents 'afs_WaitForCacheDrain' from ever becoming
true if it is not already.

Make the conditional for doing work include 'afs_WaitForCacheDrain'.

Change-Id: Id25929080eecd390d4ed56e5fffd6fe56cc8502c
Reviewed-on: http://gerrit.openafs.org/10436
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>