RFR: 8303509: Socket setTrafficClass does not work for IPv4 connections when IPv6 enabled

[JackyWoo]

This pr try to fix [JDK-8303509](https://bugs.openjdk.org/browse/JDK-8303509): Socket setTrafficClass does not work for IPv4 connections when IPv6 enabled.

### Change Log
When setting traffic class with IPv6 enabled, also set to IPv4 family.

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Commit messages:
 - add some comments
 - 8303509:fix setTrafficClass not work when IPv6 enabled

Changes: https://git.openjdk.org/jdk/pull/12852/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12852&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303509
  Stats: 6 lines in 1 file changed: 6 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/12852.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12852/head:pull/12852

PR: https://git.openjdk.org/jdk/pull/12852