RFR: 8274113: (fc) Tune FileChannel.transferFrom() [v4]

[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.

Brian Burkhalter has updated the pull request incrementally with one additional commit since the last revision:

  8274113: Remove volatile write of false to transferXXXNotSupported

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/8644/files
  - new: https://git.openjdk.java.net/jdk/pull/8644/files/6ec90b91..1ebecd1d

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8644&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8644&range=02-03

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 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