RFR: 8320971: Use BufferedInputStream.buf directly when param of implTransferTo() is trusted

[Sergey Tsypanov]

It looks like we can skip copying of `byte[]` in `BufferedInputStream.implTransferTo()` for `OutputStreams` residing in `java.io`.

See comment by @vlsi in https://github.com/openjdk/jdk/pull/10525/files#diff-e19c508d1bb6ee78697ecca66947c395adda0d9c49a85bf696e677ecbd977af1R612

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Commit messages:
 - 8320971: Use BufferedInputStream.class.getPackageName()
 - 8320971: Use BufferedInputStream.buf directly when param of implTransferTo() is trusted

Changes: https://git.openjdk.org/jdk/pull/16879/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16879&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8320971
  Stats: 8 lines in 1 file changed: 5 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/16879.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16879/head:pull/16879

PR: https://git.openjdk.org/jdk/pull/16879