tun: flag the device as supporting FMODE_NOWAIT

tun already checks for both O_NONBLOCK and IOCB_NOWAIT in its read
and write iter handlers, so it's fully ready for FMODE_NOWAIT. But
for some reason it doesn't set it. Rectify that oversight.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
Link: https://lore.kernel.org/r/3f7dc1f0-79ca-d85c-4d16-8c12c5bd492d@kernel.dk


Signed-off-by: Jakub Kicinski <kuba@kernel.org>