s390: trim ancient junk from copy_thread()

Setting and ->psw.addr in childregs of kernel thread is a rudiment of
the old kernel_thread()/kernel_execve() implementation.  Mainline hadn't
been using them since 2012.

And clarify the assignments to frame->sf.gprs - the array stores
grp6..gpr15 values to be set by __switch_to(), so frame->sf.gprs[5]
actually affects grp11, etc.

Better spell that as frame->sf.gprs[11 - 6]...
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Link: https://lore.kernel.org/r/ZAU6BYFisE8evmYf@ZenIVSigned-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/kernel/process.c

@@ -136,12 +136,12 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
 	p->thread.last_break = 1;
 
 	frame->sf.back_chain = 0;
-	frame->sf.gprs[5] = (unsigned long)frame + sizeof(struct stack_frame);
-	frame->sf.gprs[6] = (unsigned long)p;
+	frame->sf.gprs[11 - 6] = (unsigned long)&frame->childregs;
+	frame->sf.gprs[12 - 6] = (unsigned long)p;
 	/* new return point is ret_from_fork */
-	frame->sf.gprs[8] = (unsigned long)ret_from_fork;
+	frame->sf.gprs[14 - 6] = (unsigned long)ret_from_fork;
 	/* fake return stack for resume(), don't go back to schedule */
-	frame->sf.gprs[9] = (unsigned long)frame;
+	frame->sf.gprs[15 - 6] = (unsigned long)frame;
 
 	/* Store access registers to kernel stack of new process. */
 	if (unlikely(args->fn)) {
@@ -149,8 +149,6 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
 		memset(&frame->childregs, 0, sizeof(struct pt_regs));
 		frame->childregs.psw.mask = PSW_KERNEL_BITS | PSW_MASK_IO |
 					    PSW_MASK_EXT | PSW_MASK_MCHECK;
-		frame->childregs.psw.addr =
-				(unsigned long)__ret_from_fork;
 		frame->childregs.gprs[9] = (unsigned long)args->fn;
 		frame->childregs.gprs[10] = (unsigned long)args->fn_arg;
 		frame->childregs.orig_gpr2 = -1;