fs: allocate FSReqPromise stat arrays lazily #7887

Job	Run time
${{ matrix.system }}: with shared libraries	0s
github.event_name == 'workflow_dispatch' && 'Skipped job' \|\| 'Build slim tarball'	-7s
aarch64-linux: Cache V8 build	-8s
aarch64-linux: with shared ${{ matrix.openssl.name }}	-8s
	-23s

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