mac80211: do not convert to A-MSDU if frag/subframe limited

Do not start to aggregate packets in a A-MSDU frame (converting the
first subframe to A-MSDU, adding the header) if max_tx_fragments or
max_amsdu_subframes limits are already exceeded by it. In particular,
this happens when drivers set the limit to 1 to avoid A-MSDUs at all.
Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
[reword commit message to be more precise]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/tx.c

@@ -3208,9 +3208,6 @@ static bool ieee80211_amsdu_aggregate(struct ieee80211_sub_if_data *sdata,
 	if (skb->len + head->len > max_amsdu_len)
 		goto out;
 
-	if (!ieee80211_amsdu_prepare_head(sdata, fast_tx, head))
-		goto out;
-
 	nfrags = 1 + skb_shinfo(skb)->nr_frags;
 	nfrags += 1 + skb_shinfo(head)->nr_frags;
 	frag_tail = &skb_shinfo(head)->frag_list;
@@ -3226,6 +3223,9 @@ static bool ieee80211_amsdu_aggregate(struct ieee80211_sub_if_data *sdata,
 	if (max_frags && nfrags > max_frags)
 		goto out;
 
+	if (!ieee80211_amsdu_prepare_head(sdata, fast_tx, head))
+		goto out;
+
 	/*
 	 * Pad out the previous subframe to a multiple of 4 by adding the
 	 * padding to the next one, that's being added. Note that head->len