RFR: 8270169: G1: Incorrect reference discovery MT degree in concurrent marking

[Albert Mingkun Yang]

Simple change of ref discovery MT degree in concurrent marking.

Test: hotspot_gc

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Commit messages:
 - cm-rp

Changes: https://git.openjdk.java.net/jdk/pull/4741/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4741&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8270169
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4741.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4741/head:pull/4741

PR: https://git.openjdk.java.net/jdk/pull/4741