afs: Introduce afs_WakeCacheWaitersIfDrained

Consolidate common code into afs_WakeCacheWaitersIfDrained().

Change-Id: I0240d7c1be6c1a8bf22eac5fd6daf909e8320615
Reviewed-on: http://gerrit.openafs.org/10432
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>