s390/ptrace: remove opencoded offsetof

Remove opencoded offsetof and use offsetof instead.
The generated code is identical before/after this change.

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