mac80211: drop unencrypted frames in mesh fwding

commit d0c22119f574b851e63360c6b8660fe9593bbc3c upstream.

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

Reported-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>