rootfs: make pivot_root(2) dance handle initramfs case #6878

Job	Run time
cross-i386	1m 34s
cross-i386 (runc_nodmz)	1m 30s
test (ubuntu-20.04, 1.22.x, rootless, -race)	7m 45s
test (ubuntu-20.04, 1.22.x, rootless)	8m 15s
test (ubuntu-20.04, 1.22.x, -race)	9m 23s
test (ubuntu-20.04, 1.22.x)	7m 13s
test (ubuntu-20.04, 1.23.x, rootless, -race)	8m 16s
test (ubuntu-20.04, 1.23.x, rootless)	7m 43s
test (ubuntu-20.04, 1.23.x, -race)	8m 30s
test (ubuntu-20.04, 1.23.x)	7m 27s
test (ubuntu-20.04, 1.23.x, criu-dev)	8m 24s
test (ubuntu-24.04, 1.22.x, rootless, -race)	10m 27s
test (ubuntu-24.04, 1.22.x, rootless)	10m 3s
test (ubuntu-24.04, 1.22.x, -race)	7m 50s
test (ubuntu-24.04, 1.22.x)	7m 5s
test (ubuntu-24.04, 1.23.x, rootless, -race)	10m 22s
test (ubuntu-24.04, 1.23.x, rootless)	10m 8s
test (ubuntu-24.04, 1.23.x, -race)	8m 13s
test (ubuntu-24.04, 1.23.x)	7m 37s
test (ubuntu-24.04, 1.23.x, runc_nodmz)	7m 42s
test (ubuntu-24.04, 1.23.x, criu-dev)	8m 32s
test (actuated-arm64-6cpu-8gb, 1.23.x, rootless)	12m 40s
test (actuated-arm64-6cpu-8gb, 1.23.x)	8m 24s
all-done	2s
	3h 5m 5s

Provide feedback