nl80211: fix beacon head validation

If the beacon head attribute (NL80211_ATTR_BEACON_HEAD)
is too short to even contain the frame control field,
we access uninitialized data beyond the buffer. Fix this
by checking the minimal required size first. We used to
do this until S1G support was added, where the fixed
data portion has a different size.

Reported-and-tested-by:  <syzbot+72b99dcf4607e8c770f3@syzkaller.appspotmail.com>
Suggested-by: Eric Dumazet <eric.dumazet@gmail.com>
Fixes: 1d47f119

 ("nl80211: correctly validate S1G beacon head")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Link: https://lore.kernel.org/r/20210408154518.d9b06d39b4ee.Iff908997b2a4067e8d456b3cb96cab9771d252b8@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>