fix memory leak in ieee80211_end_scan()

[Ricardo Mestre]

Hi,

This plugs a resource leak in ieee80211_end_scan() when background scanning
support was added to iwn(4). Noticed by Coverity ID 1473310.

OK?

Index: ieee80211_node.c
===================================================================
RCS file: /cvs/src/sys/net80211/ieee80211_node.c,v
retrieving revision 1.151
diff -u -p -u -r1.151 ieee80211_node.c
--- ieee80211_node.c    17 Sep 2018 02:34:16 -0000      1.151
+++ ieee80211_node.c    17 Sep 2018 09:24:34 -0000
@@ -1376,6 +1376,7 @@ ieee80211_end_scan(struct ifnet *ifp)
                    IEEE80211_FC0_SUBTYPE_DEAUTH,
                    IEEE80211_REASON_AUTH_LEAVE) != 0) {
                        ic->ic_flags &= ~IEEE80211_F_BGSCAN;
+                       free(arg, M_DEVBUF, sizeof(*arg));
                        return;
                }