mac80211: support A-MSDU in fast-rx

Only works if the IV was stripped from packets. Create a smaller
variant of ieee80211_rx_h_amsdu, which bypasses checks already done
within the fast-rx context.

In order to do so, update cfg80211's ieee80211_data_to_8023_exthdr()
to take the offset between header and snap.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>