With this, ReadDir will fail a tad earlier (on open rather than on
readdir syscall). This should be the only effect of this change.

Change-Id: Icf2870f47ea6c19aad29670e78ba9bfcc13c0ac3
Reviewed-on: https://go-review.googlesource.com/c/go/+/588915


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Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
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