RFR: JDK-8266797: Fix for 8266610 breaks the build on macos

Mon May 10 13:34:59 UTC 2021

On Mon, 10 May 2021 11:09:07 GMT, Vyom Tewari <vtewari at openjdk.org> wrote:

>> I am not the Mac expert, but i checked on my MacOS Laptop and i did not found "BLKGETSIZE64" defined in sys/stat.h.
>> As Alan already told that i  took the code from FileDispatcherImpl.size0 and followed the same pattern.
>
> I will fix in this PR only. i will do the changes for only "io_util_md.c".

updated the PR as suggested.

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PR: https://git.openjdk.java.net/jdk/pull/3943