rpc_pipefs: allow rpc_purge_list to take a NULL waitq pointer

In the event that we don't have a dentry for a rpc_pipefs pipe, we still
need to allow the queue_timeout job to clean out the queue. There's just
no waitq to wake up in that event.

Cc: stable@kernel.org
Reported-by: Hans de Bruin <jmdebruin@xmsnet.nl>
Reported-by: Joerg Platte <jplatte@naasa.net>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>