RFR: 8269074: (fs) Files.copy fails to copy from /proc on some linux kernel versions

[Brian Burkhalter]

On Mon, 21 Jun 2021 20:39:02 GMT, Brian Burkhalter <bpb at openjdk.org> wrote:

> On Linux, if while executing `java.nio.file.Files.copy(Path,Path,CopyOption...)` the native function `sendfile(2)` fails with `EINVAL`, fall back to transferring via user-space buffers.

Moved user-space copy into a separate C function.

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PR: https://git.openjdk.java.net/jdk/pull/4543