Coverage Tests/mount-v2: open_tree EINVAL in uns flavor

[Snorch]

We see in multiple PRs (e.g.: #2681 and #2682) that env00 (Coverage Tests) test fails with:

2025-06-21T03:18:18.4387595Z + ./test/zdtm.py run -t zdtm/static/env00 --freezecg zdtm:t --fault 137
2025-06-21T03:18:18.5195779Z userns is supported
...
2025-06-21T03:18:19.2358397Z ========================= Run zdtm/static/env00 in uns =========================
2025-06-21T03:18:19.3086196Z Start test
2025-06-21T03:18:19.3572268Z ./env00 --pidfile=env00.pid --outfile=env00.out --envname=ENV_00_TEST
...
2025-06-21T03:18:19.6194172Z =[log]=> dump/zdtm/static/env00/197/1/restore.log
2025-06-21T03:18:19.6194515Z ------------------------ grep Error ------------------------
2025-06-21T03:18:19.6195032Z b'profiling:/home/runner/work/criu/criu/images/core-mips.gcda:Cannot open'
2025-06-21T03:18:19.6195562Z b'profiling:/home/runner/work/criu/criu/images/core-x86.gcda:Cannot open'
2025-06-21T03:18:19.6196073Z b'profiling:/home/runner/work/criu/criu/images/core.gcda:Cannot open'
2025-06-21T03:18:19.6196562Z b'profiling:/home/runner/work/criu/criu/images/stats.gcda:Cannot open'
2025-06-21T03:18:19.6196954Z b'Error: ipv4: Address already assigned.'
2025-06-21T03:18:19.6197433Z b'Error: ipv6: address already assigned.'
2025-06-21T03:18:19.6197753Z b'(00.067006)      1: mnt-v2: Mount 690 is detected as dir-mount'
2025-06-21T03:18:19.6198227Z b'(00.067008)      1: mnt-v2: Create plain mountpoint /tmp/.criu.mntns.CH1Sn5/mnt-0000000690 for 690'
2025-06-21T03:18:19.6198701Z b'(00.067013)      1: mnt-v2: \tMounting unsupported @690 (0)'
2025-06-21T03:18:19.6199189Z b'(00.067015)      1: mnt-v2: \tBind /home/runner/work/criu/criu to /tmp/.criu.mntns.CH1Sn5/mnt-0000000690'
2025-06-21T03:18:19.6199928Z b'(00.067025)      1: Error (criu/mount-v2.c:319): mnt-v2: Failed to open_tree /home/runner/work/criu/criu: Invalid argument'
2025-06-21T03:18:19.6200427Z b'(00.067742) uns: calling exit_usernsd (-1, 1)'
2025-06-21T03:18:19.6200751Z b'(00.067769) uns: daemon calls 0x557ef0d92410 (222, -1, 1)'
2025-06-21T03:18:19.6201057Z b'(00.067773) uns: `- daemon exits w/ 0'
2025-06-21T03:18:19.6201325Z b'(00.071092) uns: daemon stopped'
2025-06-21T03:18:19.6201630Z b'(00.071101) Error (criu/cr-restore.c:2324): Restoring FAILED.'
2025-06-21T03:18:19.6202094Z b'(00.074514) Error (criu/cgroup.c:1998): cg: cgroupd: recv req error: No such file or directory'
2025-06-21T03:18:19.6202543Z ------------------------ ERROR OVER ------------------------
2025-06-21T03:18:19.6202916Z ################# Test zdtm/static/env00 FAIL at CRIU restore ##################

From code I see two reasons why it can happen:

1. /home/runner/work/criu/criu is on unbindable mount
2. /home/runner/work/criu/criu has locked chidren mounts

Not sure why in env00 we have external mounts at all, but that is a secondary question.

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