mac80211: mesh: fix HWMP sequence numbering to follow standard

IEEE 802.11-2016 14.10.8.3 HWMP sequence numbering says:
If it is a target mesh STA, it shall update its own HWMP SN to
maximum (current HWMP SN, target HWMP SN in the PREQ element) + 1
immediately before it generates a PREP element in response to a
PREQ element.
Signed-off-by: Yuan-Chi Pang <fu3mo6goo@gmail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/mesh_hwmp.c

@@ -572,6 +572,10 @@ static void hwmp_preq_frame_process(struct ieee80211_sub_if_data *sdata,
 		forward = false;
 		reply = true;
 		target_metric = 0;
+
+		if (SN_GT(target_sn, ifmsh->sn))
+			ifmsh->sn = target_sn;
+
 		if (time_after(jiffies, ifmsh->last_sn_update +
 					net_traversal_jiffies(sdata)) ||
 		    time_before(jiffies, ifmsh->last_sn_update)) {