Merge pull request #20744 from yuwata/udev-netlink

udev: use netlink more aggressively

I'm pasting the comment from https://github.com/systemd/systemd/pull/20744#issuecomment-934485287
which is quite informative. The code wasn't changed significantly since then:

atenart commented 6 days ago:
> I ran tests without (93caec7) and with this PR (06735f2) on Fedora, having a few udev rules
> using attributes eligible to be cached and creating 50 veth on 4 CPUs. Although the time spent
> running the test is variable between runs, I generally saw an improvement when using this PR, e.g:
>
> 249-910-g93caec7:
> real	0m3.691s
> user	0m0.022s
> sys	0m1.338s
>
> 249-920-g06735f2:
> real	0m2.950s
> user	0m0.005s
> sys	0m0.399s
>
> On a different system than the one used above, I even saw a 40% improvement; results depend
> on many parameters (distro, udev rules, concurrent daemons accessing sysfs, etc.).
>
> Because it's quite hard to measure the improvement here (as the kernel behave differently between
> the two test cases), I also ran tests using a modified kernel not hitting the trylock logic. There was
> an improvement with this PR as well. (Take this with a grain of salt though, as the kernel was
> modified not using patches approved upstream).