[MAC80211]: Improve sanity checks on injected packets

Michael Wu noticed that the skb length checking is not taken care of enough when
a packet is presented on the Monitor interface for injection.

This patch improves the sanity checking and removes fake offsets placed
into the skb network and transport header.

Signed-off-by: Andy Green <andy@warmcat.com>
Signed-off-by: Jiri Benc <jbenc@suse.cz>
Signed-off-by: John W. Linville <linville@tuxdriver.com>