LinuxKPI: 802.11: implement ieee80211_get_hdrlen_from_skb()

Implement ieee80211_get_hdrlen_from_skb() doing basic sanity checks
on lengths (minimal length or skb data length vs. header length).

Sponsored by: The FreeBSD Foundation

(cherry picked from commit 339119949660916aa40b1d67dbe002dd7eb0d709)