kernel/exit.c: if init dies, log a signal which killed it, if any

I just received another user's pleas for help when their init
mysteriously died.  I again explained that they need to check whether it
died because of bad instruction, a segv, or something else.  Which was
an annoying detour into writing a trivial C program to spawn his init
and print its exit code:

  http://lists.busybox.net/pipermail/busybox/2012-January/077172.html

I hear you saying "just test it under /bin/sh".  Well, the crashing init
_was_ /bin/sh.

Which prompted me to make kernel do this first step automatically.  We can
print exit code, which makes it possible to see that death was from e.g.
SIGILL without writing test programs.

[akpm@linux-foundation.org: add 0x to hex number output]
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/exit.c

@@ -711,8 +711,11 @@ static struct task_struct *find_new_reaper(struct task_struct *father)
 
 	if (unlikely(pid_ns->child_reaper == father)) {
 		write_unlock_irq(&tasklist_lock);
-		if (unlikely(pid_ns == &init_pid_ns))
-			panic("Attempted to kill init!");
+		if (unlikely(pid_ns == &init_pid_ns)) {
+			panic("Attempted to kill init! exitcode=0x%08x\n",
+				father->signal->group_exit_code ?:
+					father->exit_code);
+		}
 
 		zap_pid_ns_processes(pid_ns);
 		write_lock_irq(&tasklist_lock);