vlan: Ignore DELLINK on interfaces that exists

When hitting
> Failed to create interface wlan3.1: -23 (Too many open files in system)
> Try to remove and re-create wlan3.1
hostapd deletes the AP_VLAN interface and then recreates it. Thus the
kernel assigns the same ifidx to the new interfaces and sends DELLINK
and NEWLINK mesages.

As the DELLINK messages are processed after the struct hostapd_vlan is
added, hostapd deletes the struct hostapd_vlan entry, deconfigures the
AP_VLAN interface and leaves vlan_newlink nothing to find afterwards.

So this patch makes DELLINK messages to be ignored when the interface
exists.

Signed-off-by: Michael Braun <michael-dev@fami-braun.de>
diff --git a/src/ap/vlan_init.c b/src/ap/vlan_init.c
index dc65019..2499bf3 100644
--- a/src/ap/vlan_init.c
+++ b/src/ap/vlan_init.c
@@ -651,6 +651,11 @@
 
 	if (!ifname[0])
 		return;
+	if (del && if_nametoindex(ifname)) {
+		 /* interface still exists, race condition ->
+		  * iface has just been recreated */
+		return;
+	}
 
 	wpa_printf(MSG_DEBUG,
 		   "VLAN: RTM_%sLINK: ifi_index=%d ifname=%s ifi_family=%d ifi_flags=0x%x (%s%s%s%s)",