sched: address a potential NULL pointer dereference in the GRED scheduler.

If kzalloc in gred_init returns a NULL pointer, the code follows the
error handling path, invoking gred_destroy. This, in turn, calls
gred_offload, where memset could receive a NULL pointer as input,
potentially leading to a kernel crash.

When table->opt is NULL in gred_init(), gred_change_table_def()
is not called yet, so it is not necessary to call ->ndo_setup_tc()
in gred_offload().

Signed-off-by: Jun Yang <juny24602@gmail.com>
Reviewed-by: Cong Wang <xiyou.wangcong@gmail.com>
Fixes: f25c051 ("net: sched: gred: dynamically allocate tc_gred_qopt_offload")
Link: https://patch.msgid.link/20250305154410.3505642-1-juny24602@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: Jun Yang

net/sched/sch_gred.c

@@ -913,7 +913,8 @@ static void gred_destroy(struct Qdisc *sch)
 	for (i = 0; i < table->DPs; i++)
 		gred_destroy_vq(table->tab[i]);
 
-	gred_offload(sch, TC_GRED_DESTROY);
+	if (table->opt)
+		gred_offload(sch, TC_GRED_DESTROY);
 	kfree(table->opt);
 }