Remove WPA per-VLAN groups when no more stations remain

Previously, struct wpa_group was created when the first station enters
the group and the struct wpa_group was not freed when all station left
the group. This causes a problem because wpa_group will enter
FATAL_FAILURE when a wpa_group is running while the AP_VLAN interface
has already been removed.

Fix this by adding a reference counter to struct wpa_group and free a
group if it is unused.

Signed-off-by: Michael Braun <michael-dev@fami-braun.de>

src/ap/wpa_auth_i.h

@@ -169,6 +169,8 @@ struct wpa_group {
 	u8 IGTK[2][WPA_IGTK_MAX_LEN];
 	int GN_igtk, GM_igtk;
 #endif /* CONFIG_IEEE80211W */
+	/* Number of references except those in struct wpa_group->next */
+	unsigned int references;
 };