libct/cg: fix an error of cgroup path removal #6996

Job	Run time
cross-i386	1m 42s
test (ubuntu-20.04, 1.22.x, rootless, -race)	7m 13s
test (ubuntu-20.04, 1.22.x, rootless)	6m 51s
test (ubuntu-20.04, 1.22.x, -race)	8m 3s
test (ubuntu-20.04, 1.22.x)	7m 2s
test (ubuntu-20.04, 1.23.x, rootless, -race)	7m 21s
test (ubuntu-20.04, 1.23.x, rootless)	7m 15s
test (ubuntu-20.04, 1.23.x, -race)	7m 53s
test (ubuntu-20.04, 1.23.x)	7m 14s
test (ubuntu-20.04, 1.23.x, criu-dev)	7m 39s
test (ubuntu-24.04, 1.22.x, rootless, -race)	10m 24s
test (ubuntu-24.04, 1.22.x, rootless)	9m 28s
test (ubuntu-24.04, 1.22.x, -race)	7m 34s
test (ubuntu-24.04, 1.22.x)	6m 51s
test (ubuntu-24.04, 1.23.x, rootless, -race)	9m 55s
test (ubuntu-24.04, 1.23.x, rootless)	9m 50s
test (ubuntu-24.04, 1.23.x, -race)	8m 52s
test (ubuntu-24.04, 1.23.x)	7m 44s
test (ubuntu-24.04, 1.23.x, criu-dev)	7m 54s
test (actuated-arm64-6cpu-8gb, 1.23.x, rootless)	11m 31s
test (actuated-arm64-6cpu-8gb, 1.23.x)	7m 51s
all-done	1s
	2h 46m 8s

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