Btrfs: properly check free space for tree balancing

btrfs_insert_empty_items takes the space needed by the btrfs_item
structure into account when calculating the required free space.

So the tree balancing code shouldn't add sizeof(struct btrfs_item)
to the size when checking the free space. This patch removes these
superfluous additions.

Signed-off-by: Yan Zheng <zheng.yan@oracle.com>