x86/stackframe/32: Provide consistent pt_regs

Currently pt_regs on x86_32 has an oddity in that kernel regs
(!user_mode(regs)) are short two entries (esp/ss). This means that any
code trying to use them (typically: regs->sp) needs to jump through
some unfortunate hoops.

Change the entry code to fix this up and create a full pt_regs frame.

This then simplifies various trampolines in ftrace and kprobes, the
stack unwinder, ptrace, kdump and kgdb.

Much thanks to Josh for help with the cleanups!

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Josh Poimboeuf <jpoimboe@redhat.com>
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/crash.c

@@ -73,14 +73,6 @@ static inline void cpu_crash_vmclear_loaded_vmcss(void)
 
 static void kdump_nmi_callback(int cpu, struct pt_regs *regs)
 {
-#ifdef CONFIG_X86_32
-	struct pt_regs fixed_regs;
-
-	if (!user_mode(regs)) {
-		crash_fixup_ss_esp(&fixed_regs, regs);
-		regs = &fixed_regs;
-	}
-#endif
 	crash_save_cpu(regs, cpu);
 
 	/*