RFR: 8286734: (fc) FileChannelImpl#map() cleanup after merge of Foreign Function & Memory API

[Brian Burkhalter]

Make `map()` methods more consistent.

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Commit messages:
 - 8286734: (fc) FileChannelImpl#map() cleanup after merge of Foreign Function & Memory API

Changes: https://git.openjdk.java.net/jdk/pull/8741/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8741&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8286734
  Stats: 12 lines in 1 file changed: 4 ins; 0 del; 8 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8741.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8741/head:pull/8741

PR: https://git.openjdk.java.net/jdk/pull/8741