s390/uaccess: Shorten raw_copy_from_user() / raw_copy_to_user() inline assemblies

s390/uaccess: Shorten raw_copy_from_user() / raw_copy_to_user() inline assemblies

Add specific exception handler for copy_to_user() / copy_from_user()
mvcos fault handling, which allows to shorten the inline assemblies to
three instructions.

On fault the exception handler adjusts the length used by the mvcos
instruction in a way that the instruction completes with condition code
zero, indicating the number of bytes copied with the input/output operand
'size'. This allows to calculate and return the number of bytes not copied,
if any, like required.

Loop and return value handling is changed to C so that the compiler may
optimize the code.

Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>