erofs: make the size of read data stored in buffer_ofs

erofs: make the size of read data stored in buffer_ofs

Since now xattr_iter structures have been unified, make the size of the
read data stored in buffer_ofs.  Don't bother reusing buffer_size for
this use, which may be confusing.

This is in preparation for the following further cleanup.

Signed-off-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20230613074114.120115-4-jefflexu@linux.alibaba.com
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>