udevd: increase default number of workers

The uevent handling in udevd is not cpu hungry.
So, let's increase the default number of workers per cpu.
This decrease the number of queued uevents.

Without this commit (children_max is 16 on my laptop)
```
$ journalctl -b -u systemd-udevd.service | grep reached | wc -l
1544
```
With this commit (children_max is 30 on my laptop)
```
$ journalctl -b -u systemd-udevd.service | grep reached | wc -l
7
```

src/udev/udevd.c

@@ -1786,7 +1786,7 @@ int main(int argc, char *argv[]) {
                 arg_children_max = 8;
 
                 if (sched_getaffinity(0, sizeof(cpu_set), &cpu_set) == 0)
-                        arg_children_max += CPU_COUNT(&cpu_set) * 2;
+                        arg_children_max += CPU_COUNT(&cpu_set) * 8;
 
                 mem_limit = physical_memory() / (128LU*1024*1024);
                 arg_children_max = MAX(10U, MIN(arg_children_max, mem_limit));