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/[email protected]

Cc: Masami Hiramatsu <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: "[email protected]" <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: [email protected]
Fixes: 768ae44 ("x86/ftrace: Use text_poke()")
Signed-off-by: Steven Rostedt (Google) <[email protected]>

Author: rostedt

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;
 		}
 	}