btrfs-progs: convert: simplify insert_temp_root_item()

The function requires parameters @slot and @itemoff to record where the
next item should land.

But this is overkilled, as after inserting an item, the temporary extent
buffer will have its header nritems and the item pointer updated.  We
can use that header nritems and item pointer to get where the next item
should land.

This removes the external counter to record @slot and @itemoff.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: adam900710

convert/common.c

@@ -189,17 +189,29 @@ static int setup_temp_extent_buffer(struct extent_buffer *buf,
 	return 0;
 }
 
+static u32 get_item_offset(const struct extent_buffer *eb,
+			   const struct btrfs_mkfs_config *cfg)
+{
+	u32 slot = btrfs_header_nritems(eb);
+
+	if (slot)
+		return btrfs_item_offset(eb, slot - 1);
+	else
+		return cfg->leaf_data_size;
+}
+
 static void insert_temp_root_item(struct extent_buffer *buf,
 				  struct btrfs_mkfs_config *cfg,
-				  int *slot, u32 *itemoff, u64 objectid,
-				  u64 bytenr)
+				  u64 objectid, u64 bytenr)
 {
 	struct btrfs_root_item root_item;
 	struct btrfs_inode_item *inode_item;
 	struct btrfs_disk_key disk_key;
+	u32 slot = btrfs_header_nritems(buf);
+	u32 itemoff = get_item_offset(buf, cfg);
 
-	btrfs_set_header_nritems(buf, *slot + 1);
-	(*itemoff) -= sizeof(root_item);
+	btrfs_set_header_nritems(buf, slot + 1);
+	itemoff -= sizeof(root_item);
 	memset(&root_item, 0, sizeof(root_item));
 	inode_item = &root_item.inode;
 	btrfs_set_stack_inode_generation(inode_item, 1);
@@ -217,13 +229,12 @@ static void insert_temp_root_item(struct extent_buffer *buf,
 	btrfs_set_disk_key_objectid(&disk_key, objectid);
 	btrfs_set_disk_key_offset(&disk_key, 0);
 
-	btrfs_set_item_key(buf, &disk_key, *slot);
-	btrfs_set_item_offset(buf, *slot, *itemoff);
-	btrfs_set_item_size(buf, *slot, sizeof(root_item));
+	btrfs_set_item_key(buf, &disk_key, slot);
+	btrfs_set_item_offset(buf, slot, itemoff);
+	btrfs_set_item_size(buf, slot, sizeof(root_item));
 	write_extent_buffer(buf, &root_item,
-			    btrfs_item_ptr_offset(buf, *slot),
+			    btrfs_item_ptr_offset(buf, slot),
 			    sizeof(root_item));
-	(*slot)++;
 }
 
 /*
@@ -252,8 +263,6 @@ static int setup_temp_root_tree(int fd, struct btrfs_mkfs_config *cfg,
 				u64 dev_bytenr, u64 fs_bytenr, u64 csum_bytenr)
 {
 	struct extent_buffer *buf = NULL;
-	u32 itemoff = cfg->leaf_data_size;
-	int slot = 0;
 	int ret;
 
 	/*
@@ -271,14 +280,10 @@ static int setup_temp_root_tree(int fd, struct btrfs_mkfs_config *cfg,
 	if (ret < 0)
 		goto out;
 
-	insert_temp_root_item(buf, cfg, &slot, &itemoff,
-			      BTRFS_EXTENT_TREE_OBJECTID, extent_bytenr);
-	insert_temp_root_item(buf, cfg, &slot, &itemoff,
-			      BTRFS_DEV_TREE_OBJECTID, dev_bytenr);
-	insert_temp_root_item(buf, cfg, &slot, &itemoff,
-			      BTRFS_FS_TREE_OBJECTID, fs_bytenr);
-	insert_temp_root_item(buf, cfg, &slot, &itemoff,
-			      BTRFS_CSUM_TREE_OBJECTID, csum_bytenr);
+	insert_temp_root_item(buf, cfg, BTRFS_EXTENT_TREE_OBJECTID, extent_bytenr);
+	insert_temp_root_item(buf, cfg, BTRFS_DEV_TREE_OBJECTID, dev_bytenr);
+	insert_temp_root_item(buf, cfg, BTRFS_FS_TREE_OBJECTID, fs_bytenr);
+	insert_temp_root_item(buf, cfg, BTRFS_CSUM_TREE_OBJECTID, csum_bytenr);
 
 	ret = write_temp_extent_buffer(fd, buf, root_bytenr, cfg);
 out: