x86: use kernel_stack_pointer() in process_32.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.
Signed-off-by: H. Peter Anvin <hpa@zytor.com>

arch/x86/kernel/process_32.c

@@ -134,7 +134,7 @@ void __show_regs(struct pt_regs *regs, int all)
 		ss = regs->ss & 0xffff;
 		gs = get_user_gs(regs);
 	} else {
-		sp = (unsigned long) (&regs->sp);
+		sp = kernel_stack_pointer(regs);
 		savesegment(ss, ss);
 		savesegment(gs, gs);
 	}