RFR: 8281120: G1: Rename G1BlockOffsetTablePart::alloc_block to update_for_block

[Albert Mingkun Yang]

Simple change of renaming certain methods.

Test: build

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Commit messages:
 - g1-bot-block-rename

Changes: https://git.openjdk.java.net/jdk/pull/7325/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7325&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8281120
  Stats: 18 lines in 7 files changed: 0 ins; 0 del; 18 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7325.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7325/head:pull/7325

PR: https://git.openjdk.java.net/jdk/pull/7325