RFR: 8274113: (fc) Tune FileChannel.transferFrom()

[Brian Burkhalter]

This request proposes to change `FileChannel.transferFrom()` to add a fast path on Linux and a threshold for mapped transfers on all platforms.

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Commit messages:
 - 8274113: (fc) Tune FileChannel.transferFrom()

Changes: https://git.openjdk.java.net/jdk/pull/8644/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8644&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8274113
  Stats: 130 lines in 4 files changed: 117 ins; 0 del; 13 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8644.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8644/head:pull/8644

PR: https://git.openjdk.java.net/jdk/pull/8644