[S390] ptrace: use common code for simple peek/poke operations

arch_ptrace on s390 implements PTRACE_(PEEK|POKE)(TEXT|DATA) instead of
using using ptrace_request in kernel/ptrace.c.
The only reason is the 31bit addressing mode, where we have to unmask the
highest bit.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>