Drop fragmented 802.11 frames.

Drop fragmented 802.11 frames.

Fragmented frames were never of any practical use to us anyway, given that
our net80211 stack does not (yet?) re-assemble them.

Counter-measure against attacks where an arbitrary packet is injected in a
fragment with attacker-controlled content (via an AP which supports fragments).
See https://papers.mathyvanhoef.com/usenix2021.pdf
Section 6.8 "Treating fragments as full frames"

ok mpi@