network: reload DNS only on "up" event from NetworkManager

NetworkManager reports a bunch of events, reloading DNS at each of them
doesn't make sense and is harmful - systemd have ratelimit on service
restart.

Fixes QubesOS/qubes-issues#3135

network/qubes-nmhook

@@ -4,6 +4,7 @@
 # shellcheck source=init/functions
 . /usr/lib/qubes/init/functions
 
+if [ "$2" = "up" ]; then
     /usr/lib/qubes/qubes-setup-dnat-to-ns
 
     # FIXME: Tinyproxy does not reload DNS servers.
@@ -12,3 +13,4 @@ if under_systemd ; then
     else
         service qubes-updates-proxy try-restart
     fi
+fi