Mmap: fix grow! for non file IOs (#55849)
Fixes https://github.com/JuliaLang/julia/issues/54203
Requires #55641
Based on
https://github.com/JuliaLang/julia/pull/55641#issuecomment-2334162489
cc. @JakeZw @ronisbr
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Co-authored-by: Jameson Nash <vtjnash@gmail.com>
