sd-dhcp: don't randomly ref objects

In our API design, getter-functions don't ref objects. Calls like
foo_get_bar() will not ref 'bar'. We never do that and there is no real
reason to do it in single threaded APIs. If you need a ref-count, you
better take it yourself *BEFORE* doing anything else on the parent object
(as this might invalidate your pointer).

Right now, sd_dhcp?_get_lease() refs the lease it returns. A lot of
code-paths in systemd do not expect this and thus leak the lease
reference. Fix this by changing the API to not ref returned objects.

src/network/networkd-dhcp4.c

@@ -365,7 +365,7 @@ static int dhcp_lease_renew(sd_dhcp_client *client, Link *link) {
 
         sd_dhcp_lease_unref(link->dhcp_lease);
         link->dhcp4_configured = false;
-        link->dhcp_lease = lease;
+        link->dhcp_lease = sd_dhcp_lease_ref(lease);
 
         r = sd_dhcp_lease_get_address(lease, &address);
         if (r < 0) {
@@ -454,7 +454,7 @@ static int dhcp_lease_acquired(sd_dhcp_client *client, Link *link) {
                            "PREFIXLEN=%u", prefixlen,
                            NULL);
 
-        link->dhcp_lease = lease;
+        link->dhcp_lease = sd_dhcp_lease_ref(lease);
 
         if (link->network->dhcp_mtu) {
                 uint16_t mtu;