fs: fix flush regression from fast path #556

Job	Run time
Build slim tarball	1m 32s
x86_64-darwin: with shared libraries	54m 29s
x86_64-linux: with shared libraries	27m 24s
aarch64-linux: with shared libraries	23m 32s
aarch64-darwin: with shared libraries	24m 13s
	2h 11m 10s

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