Btrfs: skip merge part for delayed data refs

When we have data deduplication on, we'll hang on the merge part
because it needs to verify every queued delayed data refs related to
this disk offset but we may have millions refs.

And in the case of delayed data refs, we don't usually have too much
data refs to merge.

So it's safe to shut it down for data refs.
Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <clm@fb.com>

fs/btrfs/delayed-ref.c

@@ -352,6 +352,13 @@ void btrfs_merge_delayed_refs(struct btrfs_trans_handle *trans,
 	struct rb_node *node;
 	u64 seq = 0;
 
+	/*
+	 * We don't have too much refs to merge in the case of delayed data
+	 * refs.
+	 */
+	if (head->is_data)
+		return;
+
 	spin_lock(&fs_info->tree_mod_seq_lock);
 	if (!list_empty(&fs_info->tree_mod_seq_list)) {
 		struct seq_list *elem;