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>