btrfs: unify handling of return values of btrfs_insert_empty_items()

The error values returned by btrfs_insert_empty_items() are following
the common patter of 0/-errno, but some callers check for a value > 0,
which can't happen. Document that and update calls to not expect
positive values.
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ctree.c

@@ -4280,6 +4280,10 @@ void btrfs_setup_item_for_insert(struct btrfs_trans_handle *trans,
 /*
  * Given a key and some data, insert items into the tree.
  * This does all the path init required, making room in the tree if needed.
+ *
+ * Returns: 0        on success
+ *          -EEXIST  if the first key already exists
+ *          < 0      on other errors
  */
 int btrfs_insert_empty_items(struct btrfs_trans_handle *trans,
 			    struct btrfs_root *root,

fs/btrfs/file-item.c

@@ -176,7 +176,6 @@ int btrfs_insert_hole_extent(struct btrfs_trans_handle *trans,
 				      sizeof(*item));
 	if (ret < 0)
 		goto out;
-	BUG_ON(ret); /* Can't happen */
 	leaf = path->nodes[0];
 	item = btrfs_item_ptr(leaf, path->slots[0],
 			      struct btrfs_file_extent_item);
@@ -1226,8 +1225,6 @@ int btrfs_csum_file_blocks(struct btrfs_trans_handle *trans,
 				      ins_size);
 	if (ret < 0)
 		goto out;
-	if (WARN_ON(ret != 0))
-		goto out;
 	leaf = path->nodes[0];
 csum:
 	item = btrfs_item_ptr(leaf, path->slots[0], struct btrfs_csum_item);

fs/btrfs/uuid-tree.c

@@ -113,7 +113,7 @@ int btrfs_uuid_tree_add(struct btrfs_trans_handle *trans, u8 *uuid, u8 type,
 
 	ret = btrfs_insert_empty_item(trans, uuid_root, path, &key,
 				      sizeof(subid_le));
-	if (ret >= 0) {
+	if (ret == 0) {
 		/* Add an item for the type for the first time */
 		eb = path->nodes[0];
 		slot = path->slots[0];