nfsd: fix comment

Obvious comment nit.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
diff --git a/fs/nfs/callback.c b/fs/nfs/callback.c
index 2e5de77..a9f1538 100644
--- a/fs/nfs/callback.c
+++ b/fs/nfs/callback.c
@@ -153,7 +153,7 @@
 }
 
 /*
- * Kill the server process if it is not already up.
+ * Kill the server process if it is not already down.
  */
 void nfs_callback_down(void)
 {