clat: make sure the tun device doesn't yet exist

(fail with EBUSY if named device already exists)

Bug: 277682237
Test: TreeHugger, ping 1.1.1.1 on v6 only wifi on bluejay
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I71ed31e945ab80bda66637c31cddb869ab1ea750
diff --git a/service/jni/com_android_server_connectivity_ClatCoordinator.cpp b/service/jni/com_android_server_connectivity_ClatCoordinator.cpp
index dbb12ee..6078e28 100644
--- a/service/jni/com_android_server_connectivity_ClatCoordinator.cpp
+++ b/service/jni/com_android_server_connectivity_ClatCoordinator.cpp
@@ -233,7 +233,7 @@
     }
 
     struct ifreq ifr = {
-            .ifr_flags = IFF_TUN,
+            .ifr_flags = static_cast<short>(IFF_TUN | IFF_TUN_EXCL),
     };
     strlcpy(ifr.ifr_name, v4interface.c_str(), sizeof(ifr.ifr_name));