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

drivers/net/tun.c

@@ -3463,6 +3463,8 @@ static int tun_chr_open(struct inode *inode, struct file * file)
 
 	sock_set_flag(&tfile->sk, SOCK_ZEROCOPY);
 
+	/* tun groks IOCB_NOWAIT just fine, mark it as such */
+	file->f_mode |= FMODE_NOWAIT;
 	return 0;
 }