afs: CacheTruncateDaemon work until Cache Drained

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: I9a60da6db65511c8bf2391a53a6f76043f825078
Reviewed-on: http://gerrit.openafs.org/10238
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>