ftrace/x86: Add back ftrace_expected for ftrace bug reports

After someone reported a bug report with a failed modification due to the
expected value not matching what was found, it came to my attention that
the ftrace_expected is no longer set when that happens. This makes for
debugging the issue a bit more difficult.

Set ftrace_expected to the expected code before calling ftrace_bug, so
that it shows what was expected and why it failed.

Link: https://lore.kernel.org/all/CA+wXwBQ-VhK+hpBtYtyZP-NiX4g8fqRRWithFOHQW-0coQ3vLg@mail.gmail.com/
Link: https://lore.kernel.org/linux-trace-kernel/20221209105247.01d4e51d@gandalf.local.home

Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: "x86@kernel.org" <x86@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: stable@vger.kernel.org
Fixes: 768ae440 ("x86/ftrace: Use text_poke()")
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

arch/x86/kernel/ftrace.c

@@ -217,7 +217,9 @@ void ftrace_replace_code(int enable)
 
 		ret = ftrace_verify_code(rec->ip, old);
 		if (ret) {
+			ftrace_expected = old;
 			ftrace_bug(ret, rec);
+			ftrace_expected = NULL;
 			return;
 		}
 	}