um: Move uml_postsetup in the init_thread stack

atomic_notifier_chain_register() and uml_postsetup() do call kernel code
that rely on the "current" kernel macro and a valid task_struct resp.
thread_info struct. Give those functions a valid stack by moving
uml_postsetup() in the init_thread stack. This moves enables a panic()
call in this early code to generate a valid stacktrace, instead of
crashing.
E.g. when an UML kernel is started with an initrd but too few physical
memory the panic() call get's actually processed.

Signed-off-by: Thomas Meyer <[e-mail]>
Signed-off-by: Richard Weinberger <[e-mail]>

arch/um/include/shared/as-layout.h

···
 extern unsigned long host_task_size;
 
 extern int linux_main(int argc, char **argv);
+extern void uml_finishsetup(void);
 
 struct siginfo;
 extern void (*sig_info[])(int, struct siginfo *si, struct uml_pt_regs *);

arch/um/kernel/um_arch.c

···
 	.priority 		= 0
 };
 
+void uml_finishsetup(void)
+{
+	atomic_notifier_chain_register(&panic_notifier_list,
+				       &panic_exit_notifier);
+
+	uml_postsetup();
+
+	new_thread_handler();
+}
+
 /* Set during early boot */
 unsigned long task_size;
 EXPORT_SYMBOL(task_size);
···
 	if (virtmem_size < physmem_size)
 		printf("Kernel virtual memory size shrunk to %lu bytes\n",
 		       virtmem_size);
-
-	atomic_notifier_chain_register(&panic_notifier_list,
-				       &panic_exit_notifier);
-
-	uml_postsetup();
 
 	stack_protections((unsigned long) &init_thread_info);
 	os_flush_stdout();

arch/um/os-Linux/skas/process.c

···
 	n = setjmp(initial_jmpbuf);
 	switch (n) {
 	case INIT_JMP_NEW_THREAD:
-		(*switch_buf)[0].JB_IP = (unsigned long) new_thread_handler;
+		(*switch_buf)[0].JB_IP = (unsigned long) uml_finishsetup;
 		(*switch_buf)[0].JB_SP = (unsigned long) stack +
 			UM_THREAD_SIZE - sizeof(void *);
 		break;