x86/ptrace: fix up botched merge of spectrev1 fix

BugLink: https://bugs.launchpad.net/bugs/1845036

I incorrectly merged commit 31a2fbb3 ("x86/ptrace: Fix possible
spectre-v1 in ptrace_get_debugreg()") when backporting it, as was
graciously pointed out at
https://grsecurity.net/teardown_of_a_failed_linux_lts_spectre_fix.php

Resolve the upstream difference with the stable kernel merge to properly
protect things.
Reported-by: Brad Spengler <spender@grsecurity.net>
Cc: Dianzhang Chen <dianzhangchen0@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: <bp@alien8.de>
Cc: <hpa@zytor.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Connor Kuehl <connor.kuehl@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

arch/x86/kernel/ptrace.c

@@ -698,11 +698,10 @@ static unsigned long ptrace_get_debugreg(struct task_struct *tsk, int n)
 {
 	struct thread_struct *thread = &tsk->thread;
 	unsigned long val = 0;
-	int index = n;
 
 	if (n < HBP_NUM) {
+		int index = array_index_nospec(n, HBP_NUM);
 		struct perf_event *bp = thread->ptrace_bps[index];
-		index = array_index_nospec(index, HBP_NUM);
 
 		if (bp)
 			val = bp->hw.info.address;