RFR: 8278268 - (ch) InputStream returned by Channels.newInputStream should have fast path for FileChannel targets [v22]

[Markus KARG]

> This sub-issue defines the work to be done to implement JDK-8265891 solely for the particular case of FileChannel.transferFrom(ReadableByteChannel), including special treatment of SelectableByteChannel.

Markus KARG has updated the pull request incrementally with one additional commit since the last revision:

  Dealing with FileChannel::write returning a value less than rndBytes.length

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Changes:
  - all: https://git.openjdk.org/jdk/pull/6711/files
  - new: https://git.openjdk.org/jdk/pull/6711/files/fb198503..8b7425ba

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=6711&range=21
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=6711&range=20-21

  Stats: 7 lines in 1 file changed: 3 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/6711.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/6711/head:pull/6711

PR: https://git.openjdk.org/jdk/pull/6711