Btrfs: reserve some space for an orphan item when unlinking

In __unlink_start_trans() if we don't have enough room for a reservation we will
check to see if the unlink will free up space.  If it does that's great, but we
will still could add an orphan item, so we need to reserve enough space to add
the orphan item.  Do this and migrate the space the global reserve so it all
works out right.  Thanks,

Signed-off-by: Josef Bacik <josef@redhat.com>

fs/btrfs/inode.c

@@ -2790,7 +2790,8 @@ static struct btrfs_trans_handle *__unlink_start_trans(struct inode *dir,
 		return ERR_PTR(-ENOMEM);
 	}
 
-	trans = btrfs_start_transaction(root, 0);
+	/* 1 for the orphan item */
+	trans = btrfs_start_transaction(root, 1);
 	if (IS_ERR(trans)) {
 		btrfs_free_path(path);
 		root->fs_info->enospc_unlink = 0;
@@ -2895,6 +2896,12 @@ static struct btrfs_trans_handle *__unlink_start_trans(struct inode *dir,
 	err = 0;
 out:
 	btrfs_free_path(path);
+	/* Migrate the orphan reservation over */
+	if (!err)
+		err = btrfs_block_rsv_migrate(trans->block_rsv,
+				&root->fs_info->global_block_rsv,
+				btrfs_calc_trans_metadata_size(root, 1));
+
 	if (err) {
 		btrfs_end_transaction(trans, root);
 		root->fs_info->enospc_unlink = 0;