mac80211: add skb length sanity checking

We just found a bug in zd1211rw where it would reject
packets in the ->tx() method but leave them modified,
which would cause retransmit attempts with completely
bogus skbs, eventually leading to a panic due to not
having enough headroom in those.

This patch adds a sanity check to mac80211 to catch
such driver mistakes; in this case we warn and drop
the skb.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>