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
1 file changed