nfsd: fix comment

Obvious comment nit.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>

fs/nfs/callback.c

@@ -153,7 +153,7 @@ int nfs_callback_up(void)
 }
 
 /*
- * 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)
 {