iwlwifi: queue: bail out on invalid freeing

If we find an entry without an SKB, we currently continue, but
that will just result in an infinite loop since we won't increment
the read pointer, and will try the same thing over and over again.
Fix this.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/iwlwifi.20210122144849.abe2dedcc3ac.Ia6b03f9eeb617fd819e56dd5376f4bb8edc7b98a@changeid

drivers/net/wireless/intel/iwlwifi/queue/tx.c

@@ -840,10 +840,8 @@ void iwl_txq_gen2_unmap(struct iwl_trans *trans, int txq_id)
 			int idx = iwl_txq_get_cmd_index(txq, txq->read_ptr);
 			struct sk_buff *skb = txq->entries[idx].skb;
 
-			if (WARN_ON_ONCE(!skb))
-				continue;
-
-			iwl_txq_free_tso_page(trans, skb);
+			if (!WARN_ON_ONCE(!skb))
+				iwl_txq_free_tso_page(trans, skb);
 		}
 		iwl_txq_gen2_free_tfd(trans, txq);
 		txq->read_ptr = iwl_txq_inc_wrap(trans, txq->read_ptr);