m/n/c/n/dhcp4c/callback: Replace x.Sub(time.Now()) with time.Until(x)

Change-Id: Iedf1300986d553dbe0721b66ae2b229333cea520
Reviewed-on: https://review.monogon.dev/c/monogon/+/3013
Reviewed-by: Serge Bazanski <serge@monogon.tech>
Tested-by: Jenkins CI
diff --git a/metropolis/node/core/network/dhcp4c/callback/callback.go b/metropolis/node/core/network/dhcp4c/callback/callback.go
index 0ac0289..cc21b4f 100644
--- a/metropolis/node/core/network/dhcp4c/callback/callback.go
+++ b/metropolis/node/core/network/dhcp4c/callback/callback.go
@@ -96,7 +96,8 @@
 		}
 
 		if new != nil {
-			remainingLifetimeSecs := int(math.Ceil(new.ExpiresAt.Sub(time.Now()).Seconds()))
+
+			remainingLifetimeSecs := int(math.Ceil(time.Until(new.ExpiresAt).Seconds()))
 			newBroadcastIP := dhcpv4.GetIP(dhcpv4.OptionBroadcastAddress, new.Options)
 			if err := netlink.AddrReplace(iface, &netlink.Addr{
 				IPNet:       newNet,