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/libsystemd-network/sd-dhcp-client.c

@@ -348,7 +348,7 @@ int sd_dhcp_client_get_lease(sd_dhcp_client *client, sd_dhcp_lease **ret) {
             client->state != DHCP_STATE_REBINDING)
                 return -EADDRNOTAVAIL;
 
-        *ret = sd_dhcp_lease_ref(client->lease);
+        *ret = client->lease;
 
         return 0;
 }

src/libsystemd-network/sd-dhcp6-client.c

@@ -260,7 +260,7 @@ int sd_dhcp6_client_get_lease(sd_dhcp6_client *client, sd_dhcp6_lease **ret) {
         if (!client->lease)
                 return -ENOMSG;
 
-        *ret = sd_dhcp6_lease_ref(client->lease);
+        *ret = client->lease;
 
         return 0;
 }

src/libsystemd-network/test-dhcp-client.c

@@ -393,7 +393,6 @@ static void test_addr_acq_acquired(sd_dhcp_client *client, int event,
         if (verbose)
                 printf("  DHCP address acquired\n");
 
-        sd_dhcp_lease_unref(lease);
         sd_event_exit(e, 0);
 }
 

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;