executor: fix overlay layer limit for non-rootfs mounts #3138

Job	Run time
build (windows/amd64)	1m 38s
build (freebsd/amd64)	1m 35s
test-freebsd-amd64	16m 9s
test-windows-amd64 (containerd, ./client)	5m 8s
test-windows-amd64 (containerd, ./cmd/buildctl)	3m 6s
test-windows-amd64 (containerd, ./worker/containerd)	2m 56s
test-windows-amd64 (containerd, ./solver)	3m 20s
test-windows-amd64 (containerd, ./frontend)	3m 32s
test-windows-amd64 (containerd, ./frontend/dockerfile)	5m 32s
test-windows-amd64 (./..., 1)	13m 34s
	56m 30s

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