x86: uaccess_64: fix return value in __copy_from_user()

__copy_from_user() will return invalid value 16 when it fails to
access user space and the size is 10.

Signed-off-by: Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>