mac80211: stop using pointers as userspace cookies

Even if the pointers are really only accessible to root and used
pretty much only by wpa_supplicant, this is still not great; even
for debugging it'd be easier to have something that's easier to
read and guaranteed to never get reused.

With the recent change to make mac80211 create an ack_skb for the
mgmt-tx path this becomes possible, only the client probe method
needs to also allocate an ack_skb, and we can store the cookie in
that skb.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
diff --git a/include/net/mac80211.h b/include/net/mac80211.h
index 39e864b..7466c55 100644
--- a/include/net/mac80211.h
+++ b/include/net/mac80211.h
@@ -875,6 +875,9 @@
 			/* 4 bytes free */
 		} control;
 		struct {
+			u64 cookie;
+		} ack;
+		struct {
 			struct ieee80211_tx_rate rates[IEEE80211_TX_MAX_RATES];
 			s32 ack_signal;
 			u8 ampdu_ack_len;