QBONE Tun device: open file descriptors with `O_NONBLOCK`.

PiperOrigin-RevId: 888233571

quiche/quic/qbone/bonnet/tun_device.cc

diff --git a/quiche/quic/qbone/bonnet/tun_device.cc b/quiche/quic/qbone/bonnet/tun_device.cc
index b9c6d1b..b989467 100644
--- a/quiche/quic/qbone/bonnet/tun_device.cc
+++ b/quiche/quic/qbone/bonnet/tun_device.cc
@@ -172,9 +172,11 @@
     }
   });
 
-  if (!OpenFileDescriptor(kernel_,
-                          absl::GetFlag(FLAGS_qbone_client_tun_device_path),
-                          if_request, O_RDWR, persist_, &file_descriptor_)) {
+  // We set O_NONBLOCK for good measure, but, from observation, all write()
+  // syscalls to the device seem to be synchronous regardless.
+  if (!OpenFileDescriptor(
+          kernel_, absl::GetFlag(FLAGS_qbone_client_tun_device_path),
+          if_request, O_NONBLOCK | O_RDWR, persist_, &file_descriptor_)) {
     return false;
   }