btrfs: fix passing 0 to ERR_PTR in btrfs_search_dir_index_item()

The ret may be zero in btrfs_search_dir_index_item() and should not
passed to ERR_PTR(). Now btrfs_unlink_subvol() is the only caller to
this, reconstructed it to check ERR_PTR(-ENOENT) while ret >= 0.

This fixes smatch warnings:

fs/btrfs/dir-item.c:353
btrfs_search_dir_index_item() warn: passing zero to 'ERR_PTR'

Fixes: 9dcbe16fccbb ("btrfs: use btrfs_for_each_slot in btrfs_search_dir_index_item")
CC: [e-mail] # 6.1+
Reviewed-by: Johannes Thumshirn <[e-mail]>
Signed-off-by: Yue Haibing <[e-mail]>
Reviewed-by: David Sterba <[e-mail]>
Signed-off-by: David Sterba <[e-mail]>

fs/btrfs/dir-item.c

···
 			return di;
 	}
 	/* Adjust return code if the key was not found in the next leaf. */
-	if (ret > 0)
-		ret = 0;
+	if (ret >= 0)
+		ret = -ENOENT;
 
 	return ERR_PTR(ret);
 }

fs/btrfs/inode.c

···
 	 */
 	if (btrfs_ino(inode) == BTRFS_EMPTY_SUBVOL_DIR_OBJECTID) {
 		di = btrfs_search_dir_index_item(root, path, dir_ino, &fname.disk_name);
-		if (IS_ERR_OR_NULL(di)) {
-			if (!di)
-				ret = -ENOENT;
-			else
-				ret = PTR_ERR(di);
+		if (IS_ERR(di)) {
+			ret = PTR_ERR(di);
 			btrfs_abort_transaction(trans, ret);
 			goto out;
 		}