RFR: 8278268 - (ch) InputStream returned by Channels.newInputStream should have fast path for FileChannel targets [v14]

[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:

  Code Deduplication as requested by Lance Andersen
  
  Signed-off-by: Markus Karg <markus at headcrashing.eu>

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/6711/files
  - new: https://git.openjdk.java.net/jdk/pull/6711/files/4bbce517..b243b68d

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=6711&range=13
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=6711&range=12-13

  Stats: 519 lines in 3 files changed: 203 ins; 314 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6711.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6711/head:pull/6711

PR: https://git.openjdk.java.net/jdk/pull/6711