x86: use kernel_stack_pointer() in dumpstack.c

The way to obtain a kernel-mode stack pointer from a struct pt_regs in
32-bit mode is "subtle": the stack doesn't actually contain the stack
pointer, but rather the location where it would have been marks the
actual previous stack frame.  For clarity, use kernel_stack_pointer()
instead of coding this weirdness explicitly.

Furthermore, user_mode() is only valid when the process is known to
not run in V86 mode.  Use the safer user_mode_vm() instead.
Signed-off-by: NH. Peter Anvin <hpa@zytor.com>

arch/x86/kernel/dumpstack.c

@@ -268,11 +268,12 @@ int __kprobes __die(const char *str, struct pt_regs *regs, long err)
 
 	show_registers(regs);
 #ifdef CONFIG_X86_32
-	sp = (unsigned long) (&regs->sp);
-	savesegment(ss, ss);
-	if (user_mode(regs)) {
+	if (user_mode_vm(regs)) {
 		sp = regs->sp;
 		ss = regs->ss & 0xffff;
+	} else {
+		sp = kernel_stack_pointer(regs);
+		savesegment(ss, ss);
 	}
 	printk(KERN_EMERG "EIP: [<%08lx>] ", regs->ip);
 	print_symbol("%s", regs->ip);