btrfs: Fixed declaration of old_len

old_len is used to store the return value of btrfs_item_size_nr().
The return value of btrfs_item_size_nr() is of type u32.
To improve code correctness and avoid mixing signed and unsigned
integers I've changed old_len to be of type u32 as well.
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: Alexandru Moise <00moses.alexander00@gmail.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/root-tree.c

@@ -141,7 +141,7 @@ int btrfs_update_root(struct btrfs_trans_handle *trans, struct btrfs_root
 	int ret;
 	int slot;
 	unsigned long ptr;
-	int old_len;
+	u32 old_len;
 
 	path = btrfs_alloc_path();
 	if (!path)