os: file operations flaky on dragonfly around CL 156365

[bcmills]

See attached screenshot. The failures seem to start at https://golang.org/cl/156365 (updates #17490), but the Dragonfly builder is a bit flaky so it's hard to say for sure (#25796, #29583).

I sampled some failures, but the only connection I can see is that they all involve file operations.

CC @tklauser @ianlancetaylor @randall77

[bcmills]

Provisionally marking as release-blocker, but please remove if appropriate. (Dragonfly is not a first-class port, but it would still be pretty unpleasant to cut a release with a major regression.)

[tklauser]

/cc @tdfbsd

[timdarbydotnet]

I think it's just temporarily gone haywire. I'll fix it when I get a chance.

[timdarbydotnet]

It's OK now; you can close this.

[ianlancetaylor]

Thanks!