freezer: add delay after freeze

[kolyshkin]

I hate to keep adding those kludges (for earlier ones, see #2918, #2791, #2774)
but lately TestFreeze (and TestSystemdFreeze) from libcontainer/integration
fails a lot (see #2907).

The failure comes and goes, and is probably this is caused by a slow host
allocated for the test, and a slow VM on top of it.

To remediate, add a small sleep on every 25th iteration in between
asking the kernel to freeze and checking its status.

In the worst case scenario (failure to freeze), this adds about 0.4 ms
(40 x 10 us) to the duration of the call.

It is hard to measure how this affects CI as GHA plays a roulette when
allocating a node to run the test on, but it seems to help. With
additional debug info, I saw somewhat frequent "frozen after 24 retries"
or "frozen after 49 retries", meaning it succeeded right after the added
sleep.

While at it, rewrite/improve the comments.

Fixes: #2907.

[AkihiroSuda]

Could you update the comment to clarify this helps what

[kolyshkin]

There's a big comment above telling the whole story...

Ah, I just found it now contradicts what I say here. Will fix.

[kolyshkin]

Comments rewritten; PTAL @AkihiroSuda

[kolyshkin]

This is from a "good" VM (once you start testing, gha gives you a good VM...).

Out of 400 runs, only 147 needed more than 1 retry.

Out of those that need a retry, there are peaks at 24 and 49, meaning sleep helps.

[kir@kir-rhat runc]$ grep frozen xx | awk '{print $5}' | sort -n | uniq -c
      8 2
      7 3
      4 4
      2 5
      1 6
      2 7
      2 8
      1 10
      1 13
      1 14
      1 19
      4 22
      1 23
     85 24
      1 25
      1 29
      1 30
      1 31
      1 36
      1 39
      1 41
      1 45
     15 49
      1 50
      1 52
      1 54
      1 123

[dqminh]

LGTM