sparc64: Convert U3copy_{from,to}_user to accurate exception reporting.

[ Upstream commit ee841d0a ]

Report the exact number of bytes which have not been successfully
copied when an exception occurs, using the running remaining length.

Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>