staging: erofs: simplify return value of `xattr_foreach'

As Dan Carpenter pointed out, there is no need to propagate positive
return values back to its callers.

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>