tree-wide: Convert compare_func's to use CMP() macro wherever possible.

Looked for definitions of functions using the *_compare_func() suffix.

Tested:
- Unit tests passed (ninja -C build/ test)
- Installed this build and booted with it.

src/libsystemd-network/lldp-neighbor.c

@@ -26,22 +26,15 @@ static int lldp_neighbor_id_compare_func(const void *a, const void *b) {
         if (r != 0)
                 return r;
 
-        if (x->chassis_id_size < y->chassis_id_size)
-                return -1;
-
-        if (x->chassis_id_size > y->chassis_id_size)
-                return 1;
+        r = CMP(x->chassis_id_size, y->chassis_id_size);
+        if (r != 0)
+                return r;
 
         r = memcmp(x->port_id, y->port_id, MIN(x->port_id_size, y->port_id_size));
         if (r != 0)
                 return r;
 
-        if (x->port_id_size < y->port_id_size)
-                return -1;
-        if (x->port_id_size > y->port_id_size)
-                return 1;
-
-        return 0;
+        return CMP(x->port_id_size, y->port_id_size);
 }
 
 const struct hash_ops lldp_neighbor_id_hash_ops = {
@@ -52,13 +45,7 @@ const struct hash_ops lldp_neighbor_id_hash_ops = {
 int lldp_neighbor_prioq_compare_func(const void *a, const void *b) {
         const sd_lldp_neighbor *x = a, *y = b;
 
-        if (x->until < y->until)
-                return -1;
-
-        if (x->until > y->until)
-                return 1;
-
-        return 0;
+        return CMP(x->until, y->until);
 }
 
 _public_ sd_lldp_neighbor *sd_lldp_neighbor_ref(sd_lldp_neighbor *n) {

src/libsystemd-network/sd-dhcp-server.c

@@ -125,6 +125,7 @@ void client_id_hash_func(const void *p, struct siphash *state) {
 
 int client_id_compare_func(const void *_a, const void *_b) {
         const DHCPClientId *a, *b;
+        int r;
 
         a = _a;
         b = _b;
@@ -132,8 +133,9 @@ int client_id_compare_func(const void *_a, const void *_b) {
         assert(!a->length || a->data);
         assert(!b->length || b->data);
 
-        if (a->length != b->length)
-                return a->length < b->length ? -1 : 1;
+        r = CMP(a->length, b->length);
+        if (r != 0)
+                return r;
 
         return memcmp(a->data, b->data, a->length);
 }