pNFS/flexfiles: Don't attempt to send layoutstats if there are no entries

If the list of mirrors is empty, then don't send an RPC call.
Signed-off-by: NTrond Myklebust <trond.myklebust@primarydata.com>

fs/nfs/flexfilelayout/flexfilelayout.c

@@ -2250,6 +2250,11 @@ ff_layout_prepare_layoutstats(struct nfs42_layoutstat_args *args)
 	args->num_dev = ff_layout_mirror_prepare_stats(args,
 			&ff_layout->generic_hdr, dev_count);
 	spin_unlock(&args->inode->i_lock);
+	if (!args->num_dev) {
+		kfree(args->devinfo);
+		args->devinfo = NULL;
+		return -ENOENT;
+	}
 
 	return 0;
 }