rootfs: make pivot_root(2) dance handle initramfs case #6846

Job	Run time
cross-i386	1m 4s
cross-i386 (runc_nodmz)	1m 19s
test (ubuntu-20.04, 1.22.x, rootless, -race)	7m 32s
test (ubuntu-20.04, 1.22.x, rootless)	7m 6s
test (ubuntu-20.04, 1.22.x, -race)	7m 55s
test (ubuntu-20.04, 1.22.x)	7m 8s
test (ubuntu-20.04, 1.23.x, rootless, -race)	7m 36s
test (ubuntu-20.04, 1.23.x, rootless)	7m 8s
test (ubuntu-20.04, 1.23.x, -race)	8m 9s
test (ubuntu-20.04, 1.23.x)	7m 40s
test (ubuntu-20.04, 1.23.x, criu-dev)	8m 32s
test (ubuntu-24.04, 1.22.x, rootless, -race)	9m 39s
test (ubuntu-24.04, 1.22.x, rootless)	9m 13s
test (ubuntu-24.04, 1.22.x, -race)	8m 3s
test (ubuntu-24.04, 1.22.x)	7m 18s
test (ubuntu-24.04, 1.23.x, rootless, -race)	9m 47s
test (ubuntu-24.04, 1.23.x, rootless)	9m 36s
test (ubuntu-24.04, 1.23.x, -race)	8m 26s
test (ubuntu-24.04, 1.23.x)	8m 4s
test (ubuntu-24.04, 1.23.x, runc_nodmz)	8m 13s
test (ubuntu-24.04, 1.23.x, criu-dev)	8m 5s
test (actuated-arm64-6cpu-8gb, 1.23.x, rootless)	9m 32s
test (actuated-arm64-6cpu-8gb, 1.23.x)	4m 41s
all-done	1s
	2h 51m 47s

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