fast-get: fix a recent regression: write before mapping RO #22996

Job	Run time
manifest-check	2m 48s
LP64-WIP	3m 55s
build-linux (-d, mnfst, lnl)	4m 48s
build-linux (mnfst, imx8m imx8ulp)	5m 39s
build-linux (mnfst, tgl-h)	4m 33s
build-linux (mnfst, imx8 imx8x)	5m 3s
build-linux (mnfst, ptl)	5m 59s
build-linux (mnfst, tgl)	4m 28s
build-windows (mnfst, mtl)	9m 51s
build-windows (mnfst, imx8m imx8ulp)	11m 59s
build-windows (mnfst, imx8 imx8x)	10m 16s
build-linux (mnfst, mtl)	4m 48s
build-windows (mnfst, ptl)	10m 26s
build-windows (mnfst, tgl)	8m 12s
build-windows (mnfst, lnl)	9m 21s
build-linux (mnfst, wcl imx95)	7m 2s
build-windows (-d, mnfst, lnl)	8m 46s
build-windows (mnfst, tgl-h)	8m 56s
build-linux (mnfst, lnl)	4m 21s
compare-linux-win	26s
	2h 11m 37s

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