RFR: 8316468: os::write incorrectly handles partial write

[Man Cao]

Hello,

Could anyone review this small but important fix?
This bug was introduced in bddf48380e658df630fecad5eda40106a24b6e1c, and regressed the fix for https://bugs.openjdk.org/browse/JDK-8303937.

-Man

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Commit messages:
 - 8316468: os::write incorrectly handles partial write

Changes: https://git.openjdk.org/jdk/pull/15808/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15808&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8316468
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/15808.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15808/head:pull/15808

PR: https://git.openjdk.org/jdk/pull/15808