fstree: handle unsupported O_TMPFILE, fix #2609

[roman-khimov]

Probe it during Init.

[codecov]

Codecov Report

Merging #2624 (982ae3c) into master (e8bc002) will decrease coverage by 0.59%.
Report is 11 commits behind head on master.
The diff coverage is 68.51%.

❗ Current head 982ae3c differs from pull request most recent head f35c0c6. Consider uploading reports for the commit f35c0c6 to get more accurate results

@@            Coverage Diff             @@
##           master    #2624      +/-   ##
==========================================
- Coverage   29.62%   29.04%   -0.59%     
==========================================
  Files         416      417       +1     
  Lines       31523    31560      +37     
==========================================
- Hits         9339     9166     -173     
- Misses      21372    21591     +219     
+ Partials      812      803       -9     

Files	Coverage Δ
...d/blobovnicza-to-peapod/blobovnicza/blobovnicza.go	82.85% <ø> (ø)
cmd/blobovnicza-to-peapod/blobovnicza/control.go	73.91% <ø> (ø)
cmd/blobovnicza-to-peapod/blobovnicza/delete.go	79.16% <ø> (ø)
cmd/blobovnicza-to-peapod/blobovnicza/errors.go	100.00% <ø> (ø)
cmd/blobovnicza-to-peapod/blobovnicza/exists.go	100.00% <ø> (ø)
cmd/blobovnicza-to-peapod/blobovnicza/get.go	91.66% <ø> (ø)
cmd/blobovnicza-to-peapod/blobovnicza/id.go	0.00% <ø> (ø)
cmd/blobovnicza-to-peapod/blobovnicza/iterate.go	58.06% <ø> (ø)
cmd/blobovnicza-to-peapod/blobovnicza/put.go	83.33% <ø> (ø)
cmd/blobovnicza-to-peapod/blobovnicza/sizes.go	100.00% <ø> (ø)
... and 27 more

... and 3 files with indirect coverage changes

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