mac80211 hwsim: verify vif pointers

mac80211-hwsim is a debugging tool for mac80211, and as such
it can very well verify that mac80211 isn't passing junk to
drivers, especially the vif pointer is prone to this because
for vlan interfaces the AP interface pointer needs to be passed.
This makes mac80211-hwsim add a magic cookie to the private vif
area and verify it whenever an operation is called that gets a
vif pointer.

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