refactor init and setns process #6306

Job	Run time
cross-i386	1m 29s
cross-i386 (runc_nodmz)	1m 19s
test (ubuntu-20.04, 1.21.x, rootless, -race)	3m 54s
test (ubuntu-20.04, 1.21.x, rootless)	3m 53s
test (ubuntu-20.04, 1.21.x, -race)	3m 54s
test (ubuntu-20.04, 1.21.x)	3m 46s
test (ubuntu-20.04, 1.22.x, rootless, -race)	3m 53s
test (ubuntu-20.04, 1.22.x, rootless)	3m 53s
test (ubuntu-20.04, 1.22.x, -race)	3m 53s
test (ubuntu-20.04, 1.22.x)	3m 49s
test (ubuntu-20.04, 1.22.x, criu-dev)	3m 55s
test (ubuntu-24.04, 1.21.x, rootless, -race)	3m 53s
test (ubuntu-24.04, 1.21.x, rootless)	3m 54s
test (ubuntu-24.04, 1.21.x, -race)	3m 55s
test (ubuntu-24.04, 1.21.x)	3m 53s
test (ubuntu-24.04, 1.22.x, rootless, -race)	3m 53s
test (ubuntu-24.04, 1.22.x, rootless)	3m 53s
test (ubuntu-24.04, 1.22.x, -race)	3m 51s
test (ubuntu-24.04, 1.22.x)	3m 53s
test (ubuntu-24.04, 1.22.x, runc_nodmz)	3m 54s
test (ubuntu-24.04, 1.22.x, criu-dev)	3m 53s
test (actuated-arm64-6cpu-8gb, 1.22.x, rootless)	3m 38s
test (actuated-arm64-6cpu-8gb, 1.22.x)	3m 36s
	1h 23m 44s

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