fs: fix flush regression from fast path #552

Job	Run time
Build slim tarball	1m 32s
x86_64-linux: with shared libraries	30m 7s
aarch64-linux: with shared libraries	24m 47s
aarch64-darwin: with shared libraries	28m 30s
x86_64-darwin: with shared libraries	34m 13s
	1h 59m 9s

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