s390/exit: remove dead reference to do_exit from copy_thread

My s390 assembly is not particularly good so I have read the history
of the reference to do_exit copy_thread and have been able to
verify that do_exit is not used.

The general argument is that s390 has been changed to use the generic
kernel_thread and kernel_execve and the generic versions do not call
do_exit.  So it is strange to see a do_exit reference sitting there.

The history of the do_exit reference in s390's version of copy_thread
seems conclusive that the do_exit reference is something that lingers
and should have been removed several years ago.

Up through 8d19f15a ("[PATCH] s390 update (1/27): arch.")  the
s390 code made a call to the exit(2) system call when a kernel thread
finished.  Then kernel_thread_starter was added which branched
directly to the value in register 11 when the kernel thread finshed.
The value in register 11 was set in kernel_thread to
"regs.gprs[11] = (unsigned long) do_exit"

In commit 37fe5d41 ("s390: fold kernel_thread_helper() into
ret_from_fork()") kernel_thread_starter was moved into entry.S and
entry64.S unchanged (except for the syntax differences between inline
assemly and in the assembly file).

In commit f9a7e025 ("s390: switch to generic kernel_thread()") the
assignment to "gprs[11]" was moved into copy_thread from the old
kernel_thread.  The helper kernel_thread_starter was still being used
and was still branching to "%r11" at the end.

In commit 30dcb099 ("s390: switch to saner kernel_execve()
semantics") kernel_thread_starter was changed to unconditionally
branch to sysc_tracenogo instead to %r11 which held the value of
do_exit.  Unfortunately copy_thread was not updated to stop passing
do_exit in "gprs[11]".

In commit 56e62a73 ("s390: convert to generic entry")
kernel_thread_starter was replaced by __ret_from_fork.  And the code
still continued to pass do_exit in "gprs[11]" despite __ret_from_fork
not caring in the slightest.

Remove this dead reference to do_exit to make it clear that s390 is
not doing anything with do_exit in copy_thread.

History Tree: https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git

Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: Alexander Gordeev <agordeev@linux.ibm.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Fixes: 30dcb099 ("s390: switch to saner kernel_execve() semantics")
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Link: https://lore.kernel.org/r/20211208202532.16409-1-ebiederm@xmission.comSigned-off-by: Heiko Carstens <hca@linux.ibm.com>

arch/s390/kernel/process.c

@@ -139,7 +139,6 @@ int copy_thread(unsigned long clone_flags, unsigned long new_stackp,
 				(unsigned long)__ret_from_fork;
 		frame->childregs.gprs[9] = new_stackp; /* function */
 		frame->childregs.gprs[10] = arg;
-		frame->childregs.gprs[11] = (unsigned long)do_exit;
 		frame->childregs.orig_gpr2 = -1;
 		frame->childregs.last_break = 1;
 		return 0;