rootfs: make pivot_root(2) dance handle initramfs case #6859

Job	Run time
cross-i386	1m 6s
test (ubuntu-20.04, 1.22.x, rootless, -race)	7m 31s
cross-i386 (runc_nodmz)	1m 25s
test (ubuntu-20.04, 1.22.x, rootless)	7m 37s
test (ubuntu-20.04, 1.22.x, -race)	8m 4s
test (ubuntu-20.04, 1.22.x)	7m 21s
test (ubuntu-20.04, 1.23.x, rootless, -race)	7m 43s
test (ubuntu-20.04, 1.23.x, rootless)	7m 42s
test (ubuntu-20.04, 1.23.x, -race)	8m 28s
test (ubuntu-20.04, 1.23.x)	7m 48s
test (ubuntu-20.04, 1.23.x, criu-dev)	10m 38s
test (ubuntu-24.04, 1.22.x, rootless, -race)	9m 39s
test (ubuntu-24.04, 1.22.x, rootless)	10m 18s
test (ubuntu-24.04, 1.22.x, -race)	8m 7s
test (ubuntu-24.04, 1.22.x)	6m 54s
test (ubuntu-24.04, 1.23.x, rootless, -race)	12m 35s
test (ubuntu-24.04, 1.23.x, rootless)	9m 37s
test (ubuntu-24.04, 1.23.x, -race)	11m 36s
test (ubuntu-24.04, 1.23.x)	11m 8s
test (ubuntu-24.04, 1.23.x, runc_nodmz)	10m 57s
test (ubuntu-24.04, 1.23.x, criu-dev)	8m 4s
test (actuated-arm64-6cpu-8gb, 1.23.x, rootless)	11m 56s
test (actuated-arm64-6cpu-8gb, 1.23.x)	8m 2s
all-done	1s
	3h 14m 17s

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