support cgroup v1 mounted with noprefix #6971

Job	Run time
cross-i386	54s
test (ubuntu-20.04, 1.22.x, rootless, -race)	7m 6s
test (ubuntu-20.04, 1.22.x, rootless)	6m 56s
test (ubuntu-20.04, 1.22.x, -race)	7m 41s
test (ubuntu-20.04, 1.22.x)	7m 3s
test (ubuntu-20.04, 1.23.x, rootless, -race)	7m 15s
test (ubuntu-20.04, 1.23.x, rootless)	7m 10s
test (ubuntu-20.04, 1.23.x, -race)	7m 45s
test (ubuntu-20.04, 1.23.x)	7m 17s
test (ubuntu-20.04, 1.23.x, criu-dev)	7m 52s
test (ubuntu-24.04, 1.22.x, rootless, -race)	9m 39s
test (ubuntu-24.04, 1.22.x, rootless)	9m 34s
test (ubuntu-24.04, 1.22.x, -race)	8m 36s
test (ubuntu-24.04, 1.22.x)	7m 6s
test (ubuntu-24.04, 1.23.x, rootless, -race)	9m 42s
test (ubuntu-24.04, 1.23.x, rootless)	10m 59s
test (ubuntu-24.04, 1.23.x, -race)	8m 3s
test (ubuntu-24.04, 1.23.x)	8m 36s
test (ubuntu-24.04, 1.23.x, criu-dev)	8m 6s
test (actuated-arm64-6cpu-8gb, 1.23.x, rootless)	11m 43s
test (actuated-arm64-6cpu-8gb, 1.23.x)	7m 51s
all-done	1s
	2h 46m 55s

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