RFR: 8303934: (fc) Use splice in FileChannel::transferFrom when the source is a pipe (Linux)

[Brian Burkhalter]

On Linux, perform `transferFrom()` via the `splice(2)` system call when the source `ReadableByteChannel` is a `Pipe.SourceChannel` (`sun.nio.ch.SourceChannelImpl`).

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Commit messages:
 - 8303934: (fc) Use splice in FileChannel::transferFrom when the source is a pipe (Linux)

Changes: https://git.openjdk.org/jdk/pull/12965/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12965&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303934
  Stats: 87 lines in 4 files changed: 49 ins; 6 del; 32 mod
  Patch: https://git.openjdk.org/jdk/pull/12965.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12965/head:pull/12965

PR: https://git.openjdk.org/jdk/pull/12965