mac80211: mesh: don't invalidate SN on discovery failure

The 2012 spec mentions that path SNs can be invalid when created (see
section 13.10.8.4 table 13-9) but AFAICT never talks about invalidating
SNs. Which makes sense: if we have figured out the path to a target at a
certain SN then we want to remember that fact. Failing to do so can lead
to routing loops because if we don't have a valid SN then we have no way
of knowing whether an incoming path message leads to or away from the
target.

However currently when discovery fails we zero out mpath->flags which
clears MESH_PATH_SN_VALID. This patch fixes that so that only the
discovery relevant flags are cleared.
Signed-off-by: Alexis Green <agreen@cococorp.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/mesh_hwmp.c

@@ -1179,7 +1179,9 @@ void mesh_path_timer(unsigned long data)
 		spin_unlock_bh(&mpath->state_lock);
 		mesh_queue_preq(mpath, 0);
 	} else {
-		mpath->flags = 0;
+		mpath->flags &= ~(MESH_PATH_RESOLVING |
+				  MESH_PATH_RESOLVED |
+				  MESH_PATH_REQ_QUEUED);
 		mpath->exp_time = jiffies;
 		spin_unlock_bh(&mpath->state_lock);
 		if (!mpath->is_gate && mesh_gate_num(sdata) > 0) {