rtnetlink: fix rtnl_dump_ifinfo() error path

syzbot found that rtnl_dump_ifinfo() could return with a lock held [1]

Move code around so that rtnl_link_ops_put() and put_net()
can be called at the end of this function.

[1]
WARNING: lock held when returning to user space!
6.12.0-rc7-syzkaller-01681-g38f83a57aa8e #0 Not tainted
syz-executor399/5841 is leaving the kernel with locks still held!
1 lock held by syz-executor399/5841:
  #0: ffffffff8f46c2a0 (&ops->srcu#2){.+.+}-{0:0}, at: rcu_lock_acquire include/linux/rcupdate.h:337 [inline]
  #0: ffffffff8f46c2a0 (&ops->srcu#2){.+.+}-{0:0}, at: rcu_read_lock include/linux/rcupdate.h:849 [inline]
  #0: ffffffff8f46c2a0 (&ops->srcu#2){.+.+}-{0:0}, at: rtnl_link_ops_get+0x22/0x250 net/core/rtnetlink.c:555

Fixes: 43c7ce6 ("rtnetlink: Protect struct rtnl_link_ops with SRCU.")
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Joe Damato <jdamato@fastly.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20241121194105.3632507-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: edumazet

net/core/rtnetlink.c

@@ -2442,7 +2442,9 @@ static int rtnl_dump_ifinfo(struct sk_buff *skb, struct netlink_callback *cb)
 			tgt_net = rtnl_get_net_ns_capable(skb->sk, netnsid);
 			if (IS_ERR(tgt_net)) {
 				NL_SET_ERR_MSG(extack, "Invalid target network namespace id");
-				return PTR_ERR(tgt_net);
+				err = PTR_ERR(tgt_net);
+				netnsid = -1;
+				goto out;
 			}
 			break;
 		case IFLA_EXT_MASK:
@@ -2457,7 +2459,8 @@ static int rtnl_dump_ifinfo(struct sk_buff *skb, struct netlink_callback *cb)
 		default:
 			if (cb->strict_check) {
 				NL_SET_ERR_MSG(extack, "Unsupported attribute in link dump request");
-				return -EINVAL;
+				err = -EINVAL;
+				goto out;
 			}
 		}
 	}
@@ -2479,11 +2482,14 @@ static int rtnl_dump_ifinfo(struct sk_buff *skb, struct netlink_callback *cb)
 			break;
 	}
 
-	if (kind_ops)
-		rtnl_link_ops_put(kind_ops, ops_srcu_index);
 
 	cb->seq = tgt_net->dev_base_seq;
 	nl_dump_check_consistent(cb, nlmsg_hdr(skb));
+
+out:
+
+	if (kind_ops)
+		rtnl_link_ops_put(kind_ops, ops_srcu_index);
 	if (netnsid >= 0)
 		put_net(tgt_net);