net: Reoder rxq_idx check in __net_mp_open_rxq()

array_index_nospec() clamp the rxq_idx within the range of
[0, dev->real_num_rx_queues), move the check before it.

Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Reviewed-by: Mina Almasry <almasrymina@google.com>
Link: https://patch.msgid.link/20250624140159.3929503-1-yuehaibing@huawei.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>