rootfs: make pivot_root(2) dance handle initramfs case #6871

Job	Run time
cross-i386	51s
cross-i386 (runc_nodmz)	1m 6s
test (ubuntu-20.04, 1.22.x, rootless, -race)	7m 42s
test (ubuntu-20.04, 1.22.x, rootless)	8m 15s
test (ubuntu-20.04, 1.22.x, -race)	8m 30s
test (ubuntu-20.04, 1.22.x)	7m 19s
test (ubuntu-20.04, 1.23.x, rootless, -race)	7m 56s
test (ubuntu-20.04, 1.23.x, rootless)	7m 24s
test (ubuntu-20.04, 1.23.x, -race)	8m 30s
test (ubuntu-20.04, 1.23.x)	8m 7s
test (ubuntu-20.04, 1.23.x, criu-dev)	8m 11s
test (ubuntu-24.04, 1.22.x, rootless, -race)	10m 11s
test (ubuntu-24.04, 1.22.x, rootless)	9m 32s
test (ubuntu-24.04, 1.22.x, -race)	8m 2s
test (ubuntu-24.04, 1.22.x)	7m 13s
test (ubuntu-24.04, 1.23.x, rootless, -race)	10m 56s
test (ubuntu-24.04, 1.23.x, rootless)	10m 1s
test (ubuntu-24.04, 1.23.x, -race)	8m 27s
test (ubuntu-24.04, 1.23.x)	7m 44s
test (ubuntu-24.04, 1.23.x, runc_nodmz)	7m 36s
test (ubuntu-24.04, 1.23.x, criu-dev)	8m 17s
test (actuated-arm64-6cpu-8gb, 1.23.x, rootless)	12m 27s
test (actuated-arm64-6cpu-8gb, 1.23.x)	8m 30s
all-done	1s
	3h 2m 48s

Provide feedback