mac80211: mesh: add missing case to PERR processing

When the nexthop is unable to resolve its own nexthop it will send back a
PERR with a zero target_sn. According to section 13.10.11.4.3 step b in the
2012 standard that perr should be forwarded and the associated mpath->sn
should be incremented. Neither one of those was happening which is rather
bad because the originator was not told that packets are black holing.
Signed-off-by: Alexis Green <agreen@cococorp.com>
CC: Jesse Jones <jjones@cococorp.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/mesh_hwmp.c

@@ -736,9 +736,12 @@ static void hwmp_perr_frame_process(struct ieee80211_sub_if_data *sdata,
 		if (mpath->flags & MESH_PATH_ACTIVE &&
 		    ether_addr_equal(ta, sta->sta.addr) &&
 		    (!(mpath->flags & MESH_PATH_SN_VALID) ||
-		    SN_GT(target_sn, mpath->sn))) {
+		    SN_GT(target_sn, mpath->sn)  || target_sn == 0)) {
 			mpath->flags &= ~MESH_PATH_ACTIVE;
-			mpath->sn = target_sn;
+			if (target_sn != 0)
+				mpath->sn = target_sn;
+			else
+				mpath->sn += 1;
 			spin_unlock_bh(&mpath->state_lock);
 			if (!ifmsh->mshcfg.dot11MeshForwarding)
 				goto endperr;