staging: erofs: fix i_blocks calculation

For compressed files, i_blocks should not be calculated
by using i_size. use i_u.compressed_blocks instead.

In addition, i_blocks was miscalculated for non-compressed
files previously, fix it as well.

Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>