move ipFRAGMENT_OFFSET_BIT_MASK to FreeRTOS_IP_Private.h

[hs2gh]

Also make ipFRAGMENT_OFFSET_BIT_MASK definition available for custom
ethernet drivers supporting IP packet pre-filtering.

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[AniruddhaKanhere]

Hello @hs2gh, thank you for this PR.

I see that you would like to move some macros to IP_Private.h, however, there is another PR from a different contributor that is doing almost the same thing that your PR does. I am afraid that this similar change will constitute a merge conflict in either your PR or theirs (whichever is merged later).

My suggestion would be, (of course, if you don't mind) to remove the macro-moving changes from your PR just because the other PR has made some changes related to the macros in various files. If they were to remove the macro change from their PR, then it would seem incomplete - and most probably break some projects. Your PR is equally important and I would like you to keep the changes adding a U to some macros.

Does that seem acceptable? I am fine either way.

Thanks again,
Aniruddha

P.S., you can contribute to the other PR as well directly (after asking the PR author of course). One PR may have more than one contributor after all.

[hs2gh]

@AniruddhaKanhere All changes are included in the Don't Fragment Flags patch. #179 thanks to evpopov :)