unix: avoid setting O_NONBLOCK needlessly by checking flags beforehand

Change-Id: I227118221df469a677f3ff140ca7b94acb7f9571
Reviewed-on: https://go-review.googlesource.com/c/sys/+/517576
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Andy Pan <panjf2000@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>

unix/syscall_unix.go

@@ -549,6 +549,9 @@ func SetNonblock(fd int, nonblocking bool) (err error) {
 	if err != nil {
 		return err
 	}
+	if (flag&O_NONBLOCK != 0) == nonblocking {
+		return nil
+	}
 	if nonblocking {
 		flag |= O_NONBLOCK
 	} else {