fs: fix flush regression from fast path #559

Job	Run time
Build slim tarball	1m 33s
x86_64-linux: with shared libraries	28m 37s
aarch64-linux: with shared libraries	23m 4s
x86_64-darwin: with shared libraries	39m 29s
aarch64-darwin: with shared libraries	26m 34s
	1h 59m 17s

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