fix runc's poststart behaviour doesn't match the runtime-spec #6506

Job	Run time
cross-i386	1m 27s
cross-i386 (runc_nodmz)	1m 33s
test (ubuntu-20.04, 1.22.x, rootless, -race)	7m 25s
test (ubuntu-20.04, 1.22.x, rootless)	6m 47s
test (ubuntu-20.04, 1.22.x, -race)	8m 23s
test (ubuntu-20.04, 1.22.x)	7m 18s
test (ubuntu-20.04, 1.23.x, rootless, -race)	7m 50s
test (ubuntu-20.04, 1.23.x, rootless)	6m 42s
test (ubuntu-20.04, 1.23.x, -race)	8m 15s
test (ubuntu-20.04, 1.23.x)	7m 20s
test (ubuntu-20.04, 1.23.x, criu-dev)	7m 43s
test (ubuntu-24.04, 1.22.x, rootless, -race)	9m 45s
test (ubuntu-24.04, 1.22.x, rootless)	9m 6s
test (ubuntu-24.04, 1.22.x, -race)	8m 3s
test (ubuntu-24.04, 1.22.x)	7m 13s
test (ubuntu-24.04, 1.23.x, rootless, -race)	9m 29s
test (ubuntu-24.04, 1.23.x, rootless)	9m 1s
test (ubuntu-24.04, 1.23.x, -race)	8m 4s
test (ubuntu-24.04, 1.23.x)	7m 18s
test (ubuntu-24.04, 1.23.x, runc_nodmz)	7m 16s
test (ubuntu-24.04, 1.23.x, criu-dev)	7m 40s
test (actuated-arm64-6cpu-8gb, 1.23.x, rootless)	11m 27s
test (actuated-arm64-6cpu-8gb, 1.23.x)	8m 38s
all-done	1s
	2h 53m 44s

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