core/cgroup: fix TasksMaxScale percentage serialization (#41011)

bus_cgroup_set_tasks_max_scale() used a hand-rolled percentage format
that produced values ~10x too small (e.g., "TasksMax=4.0%" instead of
"TasksMax=40.00%").

On daemon-reload, the incorrect value was re-read, silently reducing
the effective TasksMax by ~10x and causing fork rejections on systems
with high thread counts.

Fix by using the existing PERMYRIAD macros, consistent with memory
property handlers (MemoryMax, MemoryHigh, MemoryLow, etc.).

Fixes: #41009

src/core/dbus-cgroup.c

@@ -1010,9 +1010,9 @@ static int bus_cgroup_set_tasks_max_scale(
                 *p = (CGroupTasksMax) { v, UINT32_MAX }; /* .scale is not 0, so this is interpreted as v/UINT32_MAX. */
                 unit_invalidate_cgroup(u, CGROUP_MASK_PIDS);
 
-                uint32_t scaled = DIV_ROUND_UP((uint64_t) v * 100U, (uint64_t) UINT32_MAX);
-                unit_write_settingf(u, flags, name, "%s=%" PRIu32 ".%" PRIu32 "%%", "TasksMax",
-                                    scaled / 10, scaled % 10);
+                int scaled = UINT32_SCALE_TO_PERMYRIAD(v);
+                unit_write_settingf(u, flags, name, "TasksMax=" PERMYRIAD_AS_PERCENT_FORMAT_STR,
+                                   PERMYRIAD_AS_PERCENT_FORMAT_VAL(scaled));
         }
 
         return 1;