to make users of the callout facility able to cooperate to work around the
race caused by the callout code lowering interrupt priority level when
invoking callout handlers, something which allows other code to run before
the callout handler gets to it's spl*() call.
This is to enable the workaround for the TCP code found in PR#20390 to be
applied.
This should be backed out once a more comprehensive fix can be put in
place.
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