RFR: 8274113: (fc) Tune FileChannel.transferFrom() [v2]

[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 refreshed the contents of this pull request, and previous commits have been removed. The incremental views will show differences compared to the previous content of the PR. The pull request contains one new commit since the last revision:

  8274113: (fc) Tune FileChannel.transferFrom()

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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/4ea9b945..df436ab8

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8644&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8644&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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