mac80211: drop unencrypted frames in mesh fwding

The mesh forwarding path was not checking that data
frames were protected when running an encrypted network;
add the necessary check.

Cc: stable@vger.kernel.org
Reported-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>