[S390] ptrace: dont abuse PT_PTRACED

Nobody except ptrace itself should use task->ptrace or PT_PTRACED
directly, change arch/s390/kernel/traps.c to use the helper.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>