udev-node: optimize device node symlink creation

udev-node: optimize device node symlink creation

If multiple devices requested the same device node symlink with the same
priority, then previously we read O(N^2) of files saved in
/run/udev/links.

This makes if the requested symlink already exists with equal or higher
priority, then the symlink is kept, and skip to read all existing files,
except for one related to the current device node, in /run/udev/links.
Hence, the total amount of file read becomes O(N).

This improves performance of testcase_simultaneous_events_2 added by the
previous commit about 30%.
Before (32.8 sec):
```
 ## 3 iterations start: 11:13:44.690953163
 ## 3 iterations end: 11:14:17.493974927
```
After (23.8 sec):
```
 ## 3 iterations start: 11:17:53.869938387
 ## 3 iterations end: 11:18:17.624268345
```

This is based on the idea and analysis by Franck Bui.

Replaces #25839.

Co-authored-by: Franck Bui <fbui@suse.com>