rootfs: consolidate mountpoint creation logic #6404

Job	Run time
cross-i386	1m 12s
cross-i386 (runc_nodmz)	1m 13s
test (ubuntu-20.04, 1.21.x, rootless, -race)	7m 8s
test (ubuntu-20.04, 1.21.x, rootless)	6m 57s
test (ubuntu-20.04, 1.21.x, -race)	7m 55s
test (ubuntu-20.04, 1.21.x)	7m 10s
test (ubuntu-20.04, 1.22.x, rootless, -race)	7m 7s
test (ubuntu-20.04, 1.22.x, rootless)	6m 40s
test (ubuntu-20.04, 1.22.x, -race)	8m 7s
test (ubuntu-20.04, 1.22.x)	7m 4s
test (ubuntu-20.04, 1.22.x, criu-dev)	7m 50s
test (ubuntu-24.04, 1.21.x, rootless, -race)	9m 26s
test (ubuntu-24.04, 1.21.x, rootless)	9m 8s
test (ubuntu-24.04, 1.21.x, -race)	8m 5s
test (ubuntu-24.04, 1.21.x)	7m 23s
test (ubuntu-24.04, 1.22.x, rootless, -race)	9m 45s
test (ubuntu-24.04, 1.22.x, rootless)	9m 13s
test (ubuntu-24.04, 1.22.x, -race)	7m 50s
test (ubuntu-24.04, 1.22.x)	7m 17s
test (ubuntu-24.04, 1.22.x, runc_nodmz)	7m 9s
test (ubuntu-24.04, 1.22.x, criu-dev)	7m 40s
test (actuated-arm64-6cpu-8gb, 1.22.x, rootless)	11m 48s
test (actuated-arm64-6cpu-8gb, 1.22.x)	8m 47s
	2h 51m 54s

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