RFR: 8280136: Serial: Remove unnecessary use of ExpandHeap_lock [v4]

[Albert Mingkun Yang]

On Thu, 10 Feb 2022 17:23:42 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
>
> Albert Mingkun Yang has updated the pull request incrementally with one additional commit since the last revision:
> 
>   lower case

I have moved the lock back to `mutexLocker` and used the name `PSOldGenExpand_lock`.

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