um: remove broken double fault detection

The show_stack function had some code to detect double faults. However,
the logic is wrong and it would e.g. trigger if a WARNING happened
inside an IRQ.

Remove it without trying to add a new logic. The current behaviour,
which will just fault repeatedly until the IRQ stack is used up and the
host kills UML, seems to be good enough.

Signed-off-by: Benjamin Berg <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Johannes Berg <[e-mail]>

arch/um/include/shared/os.h

···
 extern void unblock_signals(void);
 extern int um_set_signals(int enable);
 extern int um_set_signals_trace(int enable);
-extern int os_is_signal_stack(void);
 extern void deliver_alarm(void);
 extern void register_pm_wake_signal(void);
 extern void block_signals_hard(void);

arch/um/kernel/sysrq.c

···
 	struct pt_regs *segv_regs = current->thread.segv_regs;
 	int i;
 
-	if (!segv_regs && os_is_signal_stack()) {
-		pr_err("Received SIGSEGV in SIGSEGV handler,"
-				" aborting stack trace!\n");
-		return;
-	}
-
 	if (!stack)
 		stack = get_stack_pointer(task, segv_regs);
 

arch/um/os-Linux/signal.c

···
 	unblocking = false;
 }
 #endif
-
-int os_is_signal_stack(void)
-{
-	stack_t ss;
-	sigaltstack(NULL, &ss);
-
-	return ss.ss_flags & SS_ONSTACK;
-}