wifi: iwlwifi: read txq->read_ptr under lock

In the Linux kernel, the following vulnerability has been resolved: wifi: iwlwifi: read txq->read_ptr under lock If we read txq->read_ptr without lock, we can read the same value twice, then obtain the lock, and reclaim from there to two different places, but crucially reclaim the same entry twice, resulting in the WARN_ONCE() a little later. Fix that by reading txq->read_ptr under lock.