fs: fix flush regression from fast path #4385

Job	Run time
Build slim tarball	2m 1s
aarch64-darwin: with shared libraries	18m 46s
x86_64-linux: with shared libraries	22m 28s
aarch64-linux: with shared libraries	19m 14s
x86_64-darwin: with shared libraries	40m 23s
	1h 42m 52s

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