wireguard-tools: enable reload without teardown

- preserve (active) interface (at reload)

Signed-off-by: Paul Donald <newtwen+github@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/21784
Signed-off-by: Robert Marko <robimarko@gmail.com>
(cherry picked from commit 0a576dec74)
Link: https://github.com/openwrt/openwrt/pull/21840
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

package/network/utils/wireguard-tools/files/wireguard.sh

@@ -56,12 +56,10 @@ proto_wireguard_setup() {
 	config_get nohostroute "${config}" "nohostroute"
 	config_get tunlink "${config}" "tunlink"
 
-	ip link del dev "${config}" 2>/dev/null
-	ip link add dev "${config}" type wireguard
+	# Add the link only if it didn't already exist
+	ip -br link show "${config}" >/dev/null 2>&1 || ip link add dev "${config}" type wireguard
 
-	if [ "${mtu}" ]; then
-		ip link set mtu "${mtu}" dev "${config}"
-	fi
+	[ -n "${mtu}" ] && ip link set mtu "${mtu}" dev "${config}"
 
 	proto_init_update "${config}" 1