RFR: 8280136: Serial: Remove unnecessary use of ExpandHeap_lock

[Ivan Walulya]

On Tue, 18 Jan 2022 12:03:46 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

> This PR consists of two commits:
> 
> 1. remove `ExpandHeap_lock` in Serial GC code.
> 2. rename it to `ParallelExpandHeap_lock` to indicate it's Parallel-GC only.
> 
> Test: tier1-6

lgtm!

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Marked as reviewed by iwalulya (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/7124