RFR: 8278268 - (ch) InputStream returned by Channels.newInputStream should have fast path for FileChannel targets [v21]

[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:

  Abstracted testMoreThanTwoGB

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Changes:
  - all: https://git.openjdk.org/jdk/pull/6711/files
  - new: https://git.openjdk.org/jdk/pull/6711/files/6d3e1835..fb198503

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=6711&range=20
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=6711&range=19-20

  Stats: 187 lines in 3 files changed: 68 ins; 77 del; 42 mod
  Patch: https://git.openjdk.org/jdk/pull/6711.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/6711/head:pull/6711

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