cgroup: Do not emit compat message without memory limit

Previously mere MemoryLow= directive would lead to emitting the compat
message 'Applying MemoryMax=18446744073709551615 as MemoryLimit=' even
though it carries little information.

src/core/cgroup.c

@@ -1651,7 +1651,8 @@ static void cgroup_context_apply(
 
                         if (unit_has_unified_memory_config(u)) {
                                 val = c->memory_max;
-                                log_cgroup_compat(u, "Applying MemoryMax=%" PRIu64 " as MemoryLimit=", val);
+                                if (val != CGROUP_LIMIT_MAX)
+                                        log_cgroup_compat(u, "Applying MemoryMax=%" PRIu64 " as MemoryLimit=", val);
                         } else
                                 val = c->memory_limit;