mac80211: minstrel_ht: fix out-of-bound in minstrel_ht_set_best_prob_rate

Patch fixes this splat

BUG: KASAN: slab-out-of-bounds in minstrel_ht_update_stats.isra.7+0x6e1/0x9e0
[mac80211] at addr ffff8800cee640f4 Read of size 4 by task swapper/3/0

Signed-off-by: Konstantin Khlebnikov <koct9i@gmail.com>
Link: http://lkml.kernel.org/r/CALYGNiNyJhSaVnE35qS6UCGaSb2Dx1_i5HcRavuOX14oTz2P+w@mail.gmail.com
Acked-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/rc80211_minstrel_ht.c

@@ -414,15 +414,16 @@ minstrel_ht_set_best_prob_rate(struct minstrel_ht_sta *mi, u16 index)
 	    (max_tp_group != MINSTREL_CCK_GROUP))
 		return;
 
+	max_gpr_group = mg->max_group_prob_rate / MCS_GROUP_RATES;
+	max_gpr_idx = mg->max_group_prob_rate % MCS_GROUP_RATES;
+	max_gpr_prob = mi->groups[max_gpr_group].rates[max_gpr_idx].prob_ewma;
+
 	if (mrs->prob_ewma > MINSTREL_FRAC(75, 100)) {
 		cur_tp_avg = minstrel_ht_get_tp_avg(mi, cur_group, cur_idx,
 						    mrs->prob_ewma);
 		if (cur_tp_avg > tmp_tp_avg)
 			mi->max_prob_rate = index;
 
-		max_gpr_group = mg->max_group_prob_rate / MCS_GROUP_RATES;
-		max_gpr_idx = mg->max_group_prob_rate %	MCS_GROUP_RATES;
-		max_gpr_prob = mi->groups[max_gpr_group].rates[max_gpr_idx].prob_ewma;
 		max_gpr_tp_avg = minstrel_ht_get_tp_avg(mi, max_gpr_group,
 							max_gpr_idx,
 							max_gpr_prob);
@@ -431,7 +432,7 @@ minstrel_ht_set_best_prob_rate(struct minstrel_ht_sta *mi, u16 index)
 	} else {
 		if (mrs->prob_ewma > tmp_prob)
 			mi->max_prob_rate = index;
-		if (mrs->prob_ewma > mg->rates[mg->max_group_prob_rate].prob_ewma)
+		if (mrs->prob_ewma > max_gpr_prob)
 			mg->max_group_prob_rate = index;
 	}
 }