net/sched: act_api: avoid dereferencing ERR_PTR in tcf_idrinfo_destroy

syzbot reported a crash in tc_act_in_hw() during netns teardown where
tcf_idrinfo_destroy() passed an ERR_PTR(-EBUSY) value as a tc_action
pointer, leading to an invalid dereference.

Guard against ERR_PTR entries when iterating the action IDR so teardown
does not call tc_act_in_hw() on an error pointer.

Fixes: 84a7d6797e6a ("net/sched: acp_api: no longer acquire RTNL in tc_action_net_exit()")
Link: https://syzkaller.appspot.com/bug?extid=8f1c492ffa4644ff3826
Reported-by: [e-mail]
Closes: https://syzkaller.appspot.com/bug?extid=8f1c492ffa4644ff3826
Signed-off-by: Shivani Gupta <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

net/sched/act_api.c

···
 	int ret;
 
 	idr_for_each_entry_ul(idr, p, tmp, id) {
+		if (IS_ERR(p))
+			continue;
 		if (tc_act_in_hw(p) && !mutex_taken) {
 			rtnl_lock();
 			mutex_taken = true;