fs: fix flush regression from fast path #2710

Job	Run time
Build slim tarball	1m 37s
aarch64-darwin: with shared libraries	34m 41s
aarch64-linux: with shared libraries	22m 59s
x86_64-darwin: with shared libraries	35m 56s
x86_64-linux: with shared libraries	27m 27s
	2h 2m 40s

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