rootfs: make pivot_root(2) dance handle initramfs case #6906

Job	Run time
cross-i386	1m 18s
test (ubuntu-20.04, 1.22.x, rootless, -race)	8m 10s
test (ubuntu-20.04, 1.22.x, rootless)	7m 28s
test (ubuntu-20.04, 1.22.x, -race)	9m 9s
test (ubuntu-20.04, 1.22.x)	7m 52s
test (ubuntu-20.04, 1.23.x, rootless, -race)	8m 29s
test (ubuntu-20.04, 1.23.x, rootless)	8m 0s
test (ubuntu-20.04, 1.23.x, -race)	8m 43s
test (ubuntu-20.04, 1.23.x)	8m 5s
test (ubuntu-20.04, 1.23.x, criu-dev)	8m 30s
test (ubuntu-24.04, 1.22.x, rootless, -race)	10m 8s
test (ubuntu-24.04, 1.22.x, rootless)	9m 37s
test (ubuntu-24.04, 1.22.x, -race)	8m 52s
test (ubuntu-24.04, 1.22.x)	7m 22s
test (ubuntu-24.04, 1.23.x, rootless, -race)	10m 29s
test (ubuntu-24.04, 1.23.x, rootless)	9m 51s
test (ubuntu-24.04, 1.23.x, -race)	8m 31s
test (ubuntu-24.04, 1.23.x)	8m 23s
test (ubuntu-24.04, 1.23.x, criu-dev)	8m 5s
test (actuated-arm64-6cpu-8gb, 1.23.x, rootless)	12m 35s
test (actuated-arm64-6cpu-8gb, 1.23.x)	8m 54s
all-done	1s
	2h 58m 32s

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