mac80211: skip radiotap space calculation if no monitor exists

The radiotap header length "needed_headroom" is only required if we're
sending the skb to a monitor interface. Hence, move the calculation a
bit later so the calculation can be skipped if no monitor interface is
present.
Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/rx.c

@@ -378,9 +378,6 @@ ieee80211_rx_monitor(struct ieee80211_local *local, struct sk_buff *origskb,
 	 * the SKB because it has a bad FCS/PLCP checksum.
 	 */
 
-	/* room for the radiotap header based on driver features */
-	needed_headroom = ieee80211_rx_radiotap_space(local, status);
-
 	if (local->hw.flags & IEEE80211_HW_RX_INCLUDES_FCS)
 		present_fcs_len = FCS_LEN;
 
@@ -399,6 +396,9 @@ ieee80211_rx_monitor(struct ieee80211_local *local, struct sk_buff *origskb,
 		return remove_monitor_info(local, origskb);
 	}
 
+	/* room for the radiotap header based on driver features */
+	needed_headroom = ieee80211_rx_radiotap_space(local, status);
+
 	if (should_drop_frame(origskb, present_fcs_len)) {
 		/* only need to expand headroom if necessary */
 		skb = origskb;