RFR: 8286671: (fc) Modify sun.nio.ch.FileChannelImpl.map0() to accept a FileDescriptor parameter

[Brian Burkhalter]

Add a `FileDescriptor` as the first parameter of `map0()` and remove from the corresponding native code the use of `fd_chan`.

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Commit messages:
 - 8286671: (fc) Modify sun.nio.ch.FileChannelImpl.map0() to accept a FileDescriptor parameter

Changes: https://git.openjdk.java.net/jdk/pull/8689/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8689&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8286671
  Stats: 14 lines in 3 files changed: 1 ins; 8 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8689.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8689/head:pull/8689

PR: https://git.openjdk.java.net/jdk/pull/8689