mac80211_hwsim: clean up the skb before passing it back

We need to clean up the skb before we can copy it, this
is required for proper operation since the socket it is
still attached to could potentially live in a different
network namespace or so.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>