NFSD: Re-organize nfsd_file_gc_worker()

NFSD: Re-organize nfsd_file_gc_worker()

Dave opines:

IMO, there is no need to do this unnecessary work on every object
that is added to the LRU.  Changing the gc worker to always run
every 2s and check if it has work to do like so:

 static void
 nfsd_file_gc_worker(struct work_struct *work)
 {
-	nfsd_file_gc();
-	if (list_lru_count(&nfsd_file_lru))
-		nfsd_file_schedule_laundrette();
+	if (list_lru_count(&nfsd_file_lru))
+		nfsd_file_gc();
+	nfsd_file_schedule_laundrette();
 }

means that nfsd_file_gc() will be run the same way and have the same
behaviour as the current code. When the system it idle, it does a
list_lru_count() check every 2 seconds and goes back to sleep.
That's going to be pretty much unnoticable on most machines that run
NFS servers.

Suggested-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>