RFR: JDK-8263426: Reflow JfrNetworkUtilization::send_events

[Aleksey Shipilev]

SonarCloud instance reports a problem in JfrNetworkUtilization::send_events: "Identical sub-expressions on both sides of operator "-"." here:

  const JfrTickspan interval = last_sample_instant == 0 ? cur_time - cur_time : cur_time - last_sample_instant;

Note `cur_time - cur_time`. It would seem that `interval` is effectively zero when `last_sample_instant` is not yet initialized. And I see that the subsequent code checks for `interval.value() > 0` for every interface. That can be optimized a bit: when `last_sample_instant` is not available, do not enter the loop, and don't check `interval.value()`.

Additional testing:
 - [x] `jdk_jfr` on Linux x86_64

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Commit messages:
 - JDK-8263426: Reflow JfrNetworkUtilization::send_events

Changes: https://git.openjdk.java.net/jdk/pull/2935/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2935&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8263426
  Stats: 6 lines in 1 file changed: 1 ins; 1 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2935.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2935/head:pull/2935

PR: https://git.openjdk.java.net/jdk/pull/2935