RFR: 8259956: jdk.jfr.internal.ChunkInputStream#available should return the sum of remaining available bytes

[Denghui Dong]

Could I have a review of this small fix?

In the current implementation, ChunkInputStream#available only returns the available size of the current stream, which is strange for the user.

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Commit messages:
 - 8259956: jdk.jfr.internal.ChunkInputStream#available should return the sum of remaining available bytes

Changes: https://git.openjdk.java.net/jdk/pull/2138/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2138&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8259956
  Stats: 96 lines in 2 files changed: 84 ins; 3 del; 9 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2138.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2138/head:pull/2138

PR: https://git.openjdk.java.net/jdk/pull/2138