mac80211: Only process mesh config header on frames that RA_MATCH

commit 555cb715be8ef98b8ec362b23dfc254d432a35b1 upstream.

Doing otherwise is wrong, and may wreak havoc on the mpp tables,
specially if the frame is encrypted.

Reported-by: Chaoxing Lin <Chaoxing.Lin@ultra-3eti.com>
Signed-off-by: Javier Cardona <javier@cozybit.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
[bwh: Backported to 3.2: we have a large block conditional on
 IEEE80211_RX_RA_MATCH rather than a goto conditional on the opposite,
 so delete the condition]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>