[S390] remove ptrace warning on 31 bit.

A kernel compile on 31 bit gives the following warnings in ptrace.c:

arch/s390/kernel/ptrace.c: In function 'peek_user':
arch/s390/kernel/ptrace.c:207: warning: unused variable 'dummy'
arch/s390/kernel/ptrace.c: In function 'poke_user':
arch/s390/kernel/ptrace.c:315: warning: unused variable 'dummy'

Getting rid of the dummy variables removes the warnings.

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