RFR: 8299015: Ensure that HttpResponse.BodySubscribers.ofFile writes all bytes [v4]

[Chris Hegarty]

> The implementation of `BodySubscribers.ofFile` does not correctly handle the case of a short gathering write. The write should be retried until all the bytes have been written to the underlying file.
> 
> This is generally not an issue that would occur very frequently, since the `IOV_MAX` is commonly 1024 on Mac/Linux, and 16 on Windows. However, it is clearly a bug, and should be fixed.

Chris Hegarty has updated the pull request incrementally with one additional commit since the last revision:

  another Math.addExact

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/11722/files
  - new: https://git.openjdk.org/jdk/pull/11722/files/e174f7d6..26a2d768

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11722&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11722&range=02-03

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/11722.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11722/head:pull/11722

PR: https://git.openjdk.org/jdk/pull/11722