mac80211: fix skb length check in ieee80211_scan_rx()

Replace hard-coded compile-time constants for header length check
with dynamic determination based on the frame type. Otherwise, we
hit a validation WARN_ON in cfg80211 later.

Fixes: cd418ba6 ("mac80211: convert S1G beacon to scan results")
Reported-by:  <syzbot+405843667e93b9790fc1@syzkaller.appspotmail.com>
Signed-off-by: Du Cheng <ducheng2@gmail.com>
Link: https://lore.kernel.org/r/20210510041649.589754-1-ducheng2@gmail.com


[style fixes, reword commit message]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>