Both copy_to_ and _from_user return the number of bytes, that failed to
reach their destination, not the 0/-EXXX values.

Based on patch from Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: NSam Ravnborg <sam@ravnborg.org>
Acked-by: NOliver Hartkopp <oliver.hartkopp@volkswagen.de>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>