net: stmmac: fix out-of-bounds access in a selftest

GCC 12 points out that struct tc_action is smaller than
struct tcf_action:

drivers/net/ethernet/stmicro/stmmac/stmmac_selftests.c: In function ‘stmmac_test_rxp’:
drivers/net/ethernet/stmicro/stmmac/stmmac_selftests.c:1132:21: warning: array subscript ‘struct tcf_gact[0]’ is partly outside array bounds of ‘unsigned char[272]’ [-Warray-bounds]
 1132 |                 gact->tcf_action = TC_ACT_SHOT;
      |                     ^~

Fixes: ccfc639a ("net: stmmac: selftests: Add a selftest for Flexible RX Parser")
Link: https://lore.kernel.org/r/20220519004305.2109708-1-kuba@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/stmicro/stmmac/stmmac_selftests.c

@@ -1084,8 +1084,9 @@ static int stmmac_test_rxp(struct stmmac_priv *priv)
 	unsigned char addr[ETH_ALEN] = {0xde, 0xad, 0xbe, 0xef, 0x00, 0x00};
 	struct tc_cls_u32_offload cls_u32 = { };
 	struct stmmac_packet_attrs attr = { };
-	struct tc_action **actions, *act;
+	struct tc_action **actions;
 	struct tc_u32_sel *sel;
+	struct tcf_gact *gact;
 	struct tcf_exts *exts;
 	int ret, i, nk = 1;
 
@@ -1110,8 +1111,8 @@ static int stmmac_test_rxp(struct stmmac_priv *priv)
 		goto cleanup_exts;
 	}
 
-	act = kcalloc(nk, sizeof(*act), GFP_KERNEL);
-	if (!act) {
+	gact = kcalloc(nk, sizeof(*gact), GFP_KERNEL);
+	if (!gact) {
 		ret = -ENOMEM;
 		goto cleanup_actions;
 	}
@@ -1126,9 +1127,7 @@ static int stmmac_test_rxp(struct stmmac_priv *priv)
 	exts->nr_actions = nk;
 	exts->actions = actions;
 	for (i = 0; i < nk; i++) {
-		struct tcf_gact *gact = to_gact(&act[i]);
-
-		actions[i] = &act[i];
+		actions[i] = (struct tc_action *)&gact[i];
 		gact->tcf_action = TC_ACT_SHOT;
 	}
 
@@ -1152,7 +1151,7 @@ static int stmmac_test_rxp(struct stmmac_priv *priv)
 	stmmac_tc_setup_cls_u32(priv, priv, &cls_u32);
 
 cleanup_act:
-	kfree(act);
+	kfree(gact);
 cleanup_actions:
 	kfree(actions);
 cleanup_exts: