staging: erofs: add some comments for xattr subsystem

As Dan Carpenter pointed out, it is better to document what
return values of these callbacks in `struct xattr_iter_handlers'
mean and why it->ofs is increased regardless of success or
failure in `xattr_foreach'.

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