syscall: avoid setting O_NONBLOCK needlessly by checking flags beforehand
Change-Id: I097a2941f1d1a7fd98ccf1534940d03f47ac3229
Reviewed-on: https://go-review.googlesource.com/c/go/+/517675
Auto-Submit: Ian Lance Taylor <iant@google.com> Reviewed-by: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Run-TryBot: Andy Pan <panjf2000@gmail.com>