s390/uaccess: use exception handler to zero result on get_user() failure

Historically the uaccess code pre-initializes the result of get_user()
(and now also __get_kernel_nofault()) to zero and uses the result as
input parameter for inline assemblies. This is different to what most,
if not all, other architectures are doing, which set the result to
zero within the exception handler in case of a fault.

Use the new extable mechanism and handle zeroing of the result within
the exception handler in case of a fault.

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