sched,x86: Fix L2 cache mask

Currently AMD/Hygon do not populate l2c_id, this means that for SMT
enabled systems they report an L2 per thread. This is ofcourse not
true but was harmless so far.

However, since commit: 66558b730f ("sched: Add cluster scheduler
level for x86") the scheduler topology setup requires:

  SMT <= L2 <= LLC

Which leads to noisy warnings and possibly weird behaviour on affected
chips.

Therefore change the topology generation such that if l2c_id is not
populated it follows the SMT topology, thereby satisfying the
constraint.

Fixes: 66558b730f ("sched: Add cluster scheduler level for x86")
Reported-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Tom Lendacky <thomas.lendacky@amd.com>

arch/x86/kernel/smpboot.c

@@ -470,9 +470,9 @@ static bool match_l2c(struct cpuinfo_x86 *c, struct cpuinfo_x86 *o)
 {
 	int cpu1 = c->cpu_index, cpu2 = o->cpu_index;
 
-	/* Do not match if we do not have a valid APICID for cpu: */
+	/* If the arch didn't set up l2c_id, fall back to SMT */
 	if (per_cpu(cpu_l2c_id, cpu1) == BAD_APICID)
-		return false;
+		return match_smt(c, o);
 
 	/* Do not match if L2 cache id does not match: */
 	if (per_cpu(cpu_l2c_id, cpu1) != per_cpu(cpu_l2c_id, cpu2))